Without knowing your site specifically, here are some things to consider.
If there is not one thing obviously wrong, then it is multitude of smaller things that speak to quality and performance. Most of the recent updates to Google concern these issues specifically.
Site Growth: Any site must experience growth especially when it is small. For a blog, when the blog is young, it is wise to post regularly and often enough to build a considerable base within a reasonable time. 40 posts would represent about year of work for a slower developing site. 40 posts is a good start, however, assuming for the sake of discussion, that the site is about a year old, that is less than one post per week. If your site is less than a year, then more time needs to be given to Google and your site to mature.
Post Depth: Using a lousy metric for a minute, assume that each post is a typical blog post of about 300+ words, then your posts are not considered as having depth. They may be important, lean, informative, and so on, however, unless you are making larger in-depth posts, the search engines will see them as more shallow than in-depth. Do not fall into the trap of just adding more words. Some of the best posts are short and to the point. However, if all of your posts are relatively short, then search engines may see your post/site as rather shallow. Work on some more deep content at least from time to time.
Content Uniqueness: If any post is similar to others on other sites, then any search engine will not see them as being unique and offering a new perspective. This is important. Google rates pages and will measure your page against others with an estimated performance. If you share too many terms, phrases, and so on with other posts on other sites that perform fairly well, but do not offer new insights measured by additional keyword/phrases not found on similar posts, then an assumption is made. Make sure you are offering unique perspectives and not just parroting what others say. Dare to be bold and offer new ideas more than not.
Inbound Links: For any site that is relatively new meaning a year or two, then inbound links (backlinks) are extremely important. Without them search engines cannot measure importance and popularity. As well, analysis of the links become important for link source domain rank/importance, link location, link text, location on page, and so on. If linking is made from relatively low performing sites and the link text does not vary much and the link exists in a link list rather than in content, then this would effect the value of the link. Work on getting links to your pages from higher ranking site within content if at all possible.
Post Relevancy: If the posts are not relevant to search traffic, then your posts will not rank without updating of a shift in search criteria. For example, if your topic is not that people search for, you are in trouble. Your work has to be relevant.
Search Impressions/CTR: If your pages experience low number of impressions (relevancy) and the click-through rate is low, then the importance of your site and the specific pages drops. One of the top exercises that an SEO must go through for a new site is to adjust and develop content that encourages impressions and a SERP link/snippet that encourages click-throughs. I suspect that is your primary problem.
Content Freshness: I encourage posting as much as possible. You will hear about bloggers that post just once a week. These bloggers are often posting deeper content and can afford to back-off a bit. If you were to ask them, they likely will suggest posting more often for a period. The fact that any site has to give a search engine something to chew on, is important. Content is king has survived as a tactic for a reason. Update older posts from time to time to improve them as well as work to include new content on a regular basis. For blogs, think of it as working with a metronome. You should post every so often like clockwork.
Site Performance: Make sure your site is fast and returns content fast. I cannot tell you how many times I have seen sites download everything before content therefore making the content download very long. This is similar and related to the section above.
SEO in General: SEO is important and must be studied. I warn you however, that SEO is really not a complicated thing so do not over think it and do not seek more than the very simple. Do the simple well, and you will be okay. It is actually really simple to compete against the big boys even for a small site. I have done this many times. Just keep it simple and filter out the SEO bull.
Forget the Sitemap: If you have a small site, forget the sitemap. It is not necessary. It is far better to make sure that your site can be spidered quickly and easily by any search engine. Do this first. Once your site becomes sizable, meaning say over 1000 or more pages, then think about a sitemap. It still will not be necessary, but a good time to jump in.
Patience: Exercise patience. If you are losing pages in the index, it may be that the search engine is not really seeing these pages. Sure you have links, a sitemap, and so on, but the rest of the metrics are missing such as SERP performance, CTR, effective SEO, time on site, pages per visitor, read time per page, and so on. Without these metrics, any search engine which desires to have a higher quality index, may drop pages but not forget them. You will see search engines continue to spider these pages. If improvements are made, they will return.
Wrapping it up: Google is extremely forgiving, but you have to remember that it is a machine that operates on metrics. If there is nothing obviously wrong with your site, then it is likely quite a few smaller things in total that keeps your site from performing. Search engines will drop pages, but only Google will re-include them quickly if the metrics improve. It is a catch-22 scenario however. Your pages will find it hard to improve if they are not in the index, however, you must overcome this difficulty by finding users through other means. This means seeking inbound links, shares (facebook, twitter, sharethis, stumbleupon, etc.), direct readers via e-mail campaigns, and so on. In this, pages will be re-included and perform according to your ability to manage the SERPs.
This may not be the entire list that you need to consider, but it is a good start. Trust me when I tell you, that what may be wrong with your site is that it fails to perform as well as it should. Work on your site, making it the best that it can be, promote your site appropriately, encourage impressions/click-through for the pages you have indexed, and do an honest and effective job with SEO in simple ways, and you will be fine. It will just take time and work. Site performance is not automatic. It really does take work and time to develop. It is a never ending process. It is not about putting up a blog and some posts, it is about quality and performance. Even a small blog can perform extremely well.