If you're playing and you still have ~6 upgrades that need to be unlocked, you probably need to buy more coins.

There are 2 additional upgrades that unlock when you get to ~30 Copper Coins, and 2 more when you get to 30 Silver Coins. They make your coins flip faster and boost their value by 20% (and the upgrade has 20 levels for a total of 400%). I assume there's 2 for Gold Coins as well but I haven't unlocked them yet.

I also think there's probably some balancing issues here because Silver Coins at level 30 costs in the millions, but the 2 upgrades you unlock can be leveled all the way to 20 for very cheap, hinting that they probably should have unlocked a lot earlier.