Sorry to hear about your performance in the store, but I think this has been said before and needs to be said again: Making it big and generating sustainable income through the App Store as an indie is something that is pretty rare. We tend to usually hear a lot of success stories, but not stories of those who have tried and failed, and I think that has skewed the perception of people that are researching and thinking of stepping into the mobile app development market.
I popped by this thread just right before I'm heading to bed, and I won't be looking through the previous replies, analysis and suggestions that have been posted, so apologies if what I'm saying has been mentioned before.
At the first glance of Diamond Beams, both videos and screenshots, I'm gonna be brutally honest and say that the graphics are just not polished enough to attract a user's attention. It is not bad per se, but it is lacking the final push that will bring it to the level that will catch the user's eye. Now, it gives the vibe of a Flash game from the late 90s era, with a slightly Clipart-ish feel.
Your gameplay is interesting in it's own, but other games have also executed this platform feel better, like Badland and Cloudberry Kingdom
I think you need to look at the App Store and compare your apps to the top apps our there. I'm not asking you to copy the games, but just see how they have added that extra polish to make it stand out. Be it to the sound design, the art direction, the tightness of the game mechanic, user feedback loop, etc. I would say that quite a bit of time is spent just purely on pre-production and then loads of QA and iterative changes to put in trivial changes.
All this might not seem obvious to a player unless under scrutiny, but subconsciously, people will know when something isn't quite right but can't quite put their finger on it, and that's something that can sometimes irritate players and draw them away from a game.