Google doesn’t index all the urls on your site. A lot of times urls get dropped when new ones are made. I think it depends on how many backlinks you have to your site, how old the site is, and how much do you update it. If you’re site is fairly new, it’s going to be limited how many urls it will be allowed to get indexed. I guess this is called sandboxing. As your site gets older and gets more backlinks, your index will grow as well. That’s just the way it is. Urls being deindexed is normal, just don’t let this discourage you.