I'm newbie in Google Webmaster Tools and I use sitemap.xml to submit urls.
The problem is that I've submitted 150 urls and only 17 are indexed! Is that a problem or they will be indexed later?
If it's a new site, give it some time.
You can review for any crawl errors in Webmaster Tools > Health > Crawl Errors to see if there are any problems with the site preventing Google from crawling.
If the content on these other pages is not unique Google may choose not to include it in the index, too.
I have found this too, it puzzled me for ages but I think I've found what's causing it. As Gabriel said Google may not chose to include those pages into the index because of content being either not unique, not enough content or not relevant enough.
If you look under Webmaster Tools > Health > Index Status > Advanced you can see how many pages Google looked at and its "Total Indexed" amount and also its "Not Selected" amount.
The best way I've found for it to index your pages is to build links, either to the site in general or particular pages that aren't indexed; it doesn't have to be tons of links but just a couple tends to make Google see those pages as slightly more authoritative .