Hi there,
First of all, having more indexed pages is not what you should be aiming for. As its know that having more pages indexed can cause cannibalization, duplicate content and thin content problems.
In your case, I'd try to analyze why those pages are being left off by Google.
-> My guess: poor or thin content, as it is UGC.
Here some other questions that might help you work out the issue:
1- How long and how interesting is the content on those pages?
2- How many links are pointing to them?
3- Are pages with high internal traffic being left out?
4- Were any of them indexed just for a short period of time? On what query?
5- Are there "better" pages on your website that resolve user questions on Google search, rather than the UGC pages?
Hope it helps,
Best luck.
Gaston