Q: My site's PageRank has gone up / gone down / not changed in months!
A: Don't worry. In fact, don't bother thinking about it. We only update the PageRank displayed in Google Toolbar a few times a year; this is our respectful hint for you to worry less about PageRank, which is just one of over 200 signals that can affect how your site is crawled, indexed and ranked. PageRank is an easy metric to focus on, but just because it's easy doesn't mean it's useful for you as a site owner. If you're looking for metrics, we'd encourage you to check out Analytics, think about conversion rates, ROI (return on investment), relevancy, or other metrics that actually correlate to meaningful gains for your website or business.
Pagerank updates only every so often (If I recall correctly, it's currently 4 times a year). So be prepared to wait.
Luckily, pagerank isn't everything. If you have a proper site, which is valuable to users(!), google will show your site in the results.
Make sure your site has proper content, something which makes users stay on your site, maybe share on socialmedia, get fresh content and keep updating your site and the value of your site will increase.
Sitemaps, W3C errors and all those other technical things are good to have, but not the deciding factor. Content, content, content. The backend should only make it easier for bots to understand the.... Content.
If your website is older than 1/4th of a year and you still have this problem, you should try to track why there is no change. What to SEO-test sites say? Does Google webmastertools give you any hint as to why? Maybe some virus has been detected? Or malicous content was found on this domain (possibly by previous owner!).