Steam Discussion.

Others and I have noticed that sometimes cards will only drop as you exit a game, and not during. This causes the idler to idle forever, as it's still seeing that there are still card drops available. In these situations, when you close the idler, the trading card reward will show up, but not during idling. I'm going to do some more experimentation and see if there's a reason or it's just something I have to program the idler around.