This is taken from the official knowledge base:
First, check the limits below to make sure they are past their expiration date.
The time limits are as follows:
5 days - random opponent games (you created the game through random opponent)
11 days - non-random opponent (you created a game via contact list, user name or rematch or you received a request from another player)
Any game that goes LONGER than those durations is autoresigned by the server. (technically that means that a random game typically expires on day 6, but the system may occasionally fall behind, so it may take slightly longer)
If you have games that have not resigned past the appropriate time limits, then in reality they probably already expired but the games list on your device is out of sync with the server. Reinstalling will ensure that your copy of the game list is in sync with our server.
Also, we get a lot of questions regarding unexpired random opponent games that often turn out to be non-random games. Only games that were created via the Random Opponent option are truly random.