Double check your Badges page that all 15 have drops remaining. Free-to-Play games do not give you free drops just for adding them to your library, you must spend 9$US for each drop. Non-F2P games only give half (rounded up) of the number of cards in the set. Duplicates will count against this number. Trading/Selling/Using these card will not give you additional/replacement drops.

Due to changes in the refund policy, you now need 2 hours of playtime before the normal card drop system starts. Idle Master prioritizes getting all games with remaining drops up to the 2 hour mark. If you have 5 games with less than 2 hours of playtime, that's why you only have 5 games that start idling.

Once all of your games have 2 hours of playtime Idle Master will switch to idling 1 game at a time since running multiple games simultaneously at best is the same as running them sequentially. There have been reports that multiple games will completely prevent card drops, but no official word on such a change.