Let's go with quick basics.
Google has an enormous index of pages in database, no matter if they are public (indexed) or private (still there but not indexed).
Each time you try to index your page (web property) Google actually needs to make a decision is it worth to spend their time & money (crawling bots & bandwith) to revisit this page and actually index it.
This is why you need to make sure it is at least worth indexing.
Our best advice would be to keep pages that you want in indexed in "decent shape" and worthwhile of indexing for highest success rates.
Now, to "brute force" indexing In our experience even if page isn't that worth of it (social bookmark, profile page or similar...) what you need to do is to make a google crawlers revisit that same page multiple times and it will eventually get in index almost 99,9% of time.