People don't bother to send feedback because usually there's no reward for doing so. Typing feedback on a mobile phone is time consuming as well. Those two things are major barriers to the user. I don't have research to back it up; it's common sense.
When was the last time that you filled out a form or survey to give feedback for an app or site? It's probably been a while if at all.
You'll have to give tangible rewards like discounts or gift cards via a random draw. If it's the type of app of that users will regularly re-use, they'll be more emotionally invested in the app's stability, but you will still need to reward them.
To be honest, it sounds like the problem is that you're expecting users to help you bug hunt. To me that's like having a store and expecting customers to point out spills or misplaced items for you. That's your responsibility not theirs.