Matchmade games are any game that is not a custom game, so any game that is played that isn't a custom game can give you a key/keyshard. However, the reason behind you not getting a key/shard is that there is a random chance to get one after a matchmade game, but it doesn't happen every game. I have gotten one about every other game so far, so just play more matches and you will get some.

As of patch 6.6 TT and ARAM are bugged so keys won't drop from them. A fix is planned to be released in the next patch.