I'll tell you why you do not ever shall receive sales success. You are to greedy and stupid. Your products which are 15 years old in your store cost $ 20-30. What? 15 years before ??? I would not give even $ 10.
Your prices should fall by half. I understand it's your job but your game cost as some big titles on release. For example, your CMBS costs $ 60 and the Witcher III $ 30... games of the same year... and WIII is 3 class above.
The second issue is your fear for enter the open sorce engine friendly to modders. Look at games such as Men of War or Company of Heroes that are friendly to player and so successful.
In my opinion, if something does not change - you shall fall.
End