I think you have to make your thumbnail more eye-catching. Bold, bright and easy readable text, cantrast renders, that can tell the player what he can do un the game, for example:
This one literally represents you main goals of the game and its very eye-catching. No one hates to win. But adding text that says ROB and WIN is not that necessary, since your thumbnails intuitively tell the potential player what you can do.
Good luck on your project.