This post in The Signe of the Frothing Mug mirrors most of my own experiences. Sometimes it happens unexpectedly. With so many busy working schedules, family issues and other real life issues, someone misses one session. As we are in a middle of a scenario, we take the time out waiting for the player to return. Suddenly, someone else fails to show up on the second week. Before we know it, we don't play for three weeks in a row, and I begin to lose interest. After all, I'm not willing to spend time preparing a session if I don't know whether we're playing or not. And then, the campaign implodes as everyone loses interest. Some of us been there and done that. So, what can we do to avoid a campaign implosion by lack of attendance?
In the past, we arranged for a quorum, that is, if at least half the players could make it, we would still play regardless. This almost guarantees the campaign never stops. I used this with mixed results. I never got the sense I was playing with a cohesive group as different players failed to show up on different occasions. I was never playing with the full group. This meant individual storylines failed to take root and even the overall story arcs didn't generate much interest because no player was there all the time to play through it.
We also tried to play in episodic style. This style meant sacrificing long, involving and complex stories for shorter stories of 1-2 sessions. This worked better specially if there were a rotating cast. Every story started with whoever was available and off they went. At the end of session, they returned to their base camp (or ship or whatever) to rest. Next session, rinse and repeat. The problem is this only works if the group is traveling in a mobile base (a spaceship or a boat) or if the campaign is bound within certain geographic limits and the group returns to the same place to rest. In some games, requiring lots of travel, it becomes increasingly difficult to rationalize why some of the missing characters manage to find the rest of the group if they travel all the way to the other side of world.
In the end, I find that both solutions are not ideal ones. When I start a new campaign, I just hope the players commit to the game schedule we agree and take it seriously. Currently, we play twice a month. We hope that, by devoting one week to gaming and another to our families, we will maintain a regular gaming schedule and the campaign won't implode from lack of interest generated by long gaps in play. It still doesn't solve the problem of one player missing a session and waiting three weeks to be able to play again, but then again, by not imposing a weekly game, perhaps the players will show up more often because they are not forced to choose between the game and other things so often.