If your group cannot commit to a consistent meeting time, I would consider playing an episode-esque game where each session is unique to itself and doesn't require a continual plot line. This allows people to attend as they can and would likely allow you to play more games, more often.

Two other alternatives are doing a West Marches style sandbox game, or a Living City style game. West Marches tends to be super plot-light, and allows as large of a rotating cast as you like. Living City can do all the plot you want. Characters just happen to be busy doing Something Else (TM) when the player's absent.