Card Drop Algorithm Changed by Valve
It appears that the algorithm used by Steam to determine the rate at which cards drop was changed yesterday by Valve. After hearing numerous reports and doing testing of my own, it appears they have made this change to attempt to prevent people from purchasing a game, idling it for cards, selling the cards then refunding the game. As a result, most games will not start dropping Steam Trading Cards until approximately 2 hours of playtime has been recorded on your account.
As a reminder: if Idle Master is running and Steam shows you are "in-game" (and that game has available card drops) then you will eventually get those card drops. There is no difference between using Idle Master to idle the game, or downloading a game and sitting in it's main menu to idle for this purpose.
While I understand that this was a necessary change for Valve to make to help prevent fraud, I also know that it immensely affects those of us with a large number of games to idle. For me personally, this means that my idling time has increased approximately 650 hours (almost 28 days) as a result of this change.
Additionally, I encourage everyone who purchases games on Steam to test them out with your PC and be sure that you will not want to have that game refunded before using Idle Master to idle it's cards. The program includes a blacklist feature that can be employed to prevent the program from idling these games if you're not sure.
This change could be temporary. Valve could be trying to affect the supply of trading cards in the market, or it could be the result of the ongoing sale. If the situation changes further I'll try to keep everyone updated.