Ask HN: Should I develop my side project further or just let it go?

I have recently released released an iPhone chess app on the App Store. This is the evolution of a previous similar side project I developed for Android. The version for Android was appreciated beyond my expectations and this motivated me to develop the project further. Having already sold some thousand units on Android based on contents purely developed by me I thought I could do better with a more sofisticated app based on contents provided by real chess trainers. I managed to find one FM and one IM willing to colloborate and started to work on the iPhone version. I rewrote the app from scratch and launched a MVP with three volumes written by the two authors mentioned above. I expected to sell at least three times than the Android version. Nothing was further from truth: results have been extremely dissapointing. The app has been downloaded about 700 times and I sold 5 volumes in total.

Feedback that I got from users and beta-testers is generally positive as can be seen here:

http://goo.gl/pBMML8

however this appreciation does not convert into downloads. The application is based on a portable c++ core ( almost 85% of the code, the rest being objective-c ) and could be easily ported to other mobile and desktop platforms.

Is there anyone that could be interested in joining forces to develop the project further? I would appreciate very much also any kind of feedback on the project including criticisms. I am at a turning point and would like to understand If it is worth to spend further effort on the project or if it would be better to let it go.