This actually has been addressed multiple times, but since I'm here I'll answer again.

This isn't an issue with idlemaster. This is how Steam set up their card drops. You get drops when you "close" the game. The community of Idlemaster has found that the most efficient way to obtain cards is to open idlemaster, run a game for about 1-5 minutes, then click "next game". Repeat this process until you run out of valid games in your IM. Reload the list either by closing the program or by changing a game order setting. Repeat until you hit idle complete.

Then start the cycle again once you have acquired more games.

Yeah, happens occasionally that steam doesn't drop the cards until you close the game, steam issue not idle-master issue, no harm in adding it to the FAQ though.