I think the biggest issue is you switched to using credits. That and you've only let this play out for 20 days. People can take quite a while to evaluate an app and decide to purchase. You have more experimenting to do. A one-time upgrade fee for unlimited or a yearly subscription seem like a better fit for how a user would expect to pay for this type of app than credits. At the very least perhaps add an "expensive" unlimited use option and I bet you get some purchases of that.