That might be your problem too. The absolute next thing I build, after features, is a feedback pipeline. I do this before even integrating the payment gateway. Often it's in the form of some support forum or in app public chatroom.

Take a close look at any comments wherever you advertise. I'd even recommend against FB/Google advertising at first because it's difficult to get feedback.

Analytics might help too and you can see where people are dropping off. But they're quite inefficient.

Sometimes you just have email people who signed up in person to ask them. People are usually very open to giving brief feedback; just don't drag them into some focus group or 10 minute talk.

Being able to iterate based on feedback is usually the make or break point of an early stage entrepreneur.