We've had this problem to some degree with both of the campaigns I'm currently in.
First thing we do is just call out to everyone and state the obvious: "Hey guys, I notice we're having a lot of trouble meeting regularly. Is this campaign not doing it for people? Or are we having scheduling problems?"
Once we know which the problem is, we start talking about whether we want to change things to fix it, and how we would fix it. If people are just losing interest in the campaign, we discuss the problems and how to get people excited again. If scheduling is the problem, we talk about changing that: moving the day, changing the way we schedule, reducing (or increasing) the frequency. For instance, in the one I ran we had huge problems because we'd only try to schedule a session or two out and tried for biweekly. This year, we fixed the schedule at the beginning of the year to first and third Saturdays, so everyone knew from the outset what the plan was.
Then, ask everyone to commit to working towards the solutions. Literally, say "Sounds like we have a plan. Is everyone committed to making this happen?" Maybe someone will say no. That input will let the whole group know whether this is going to happen.
I've found that when one member of the group is feeling something, odds are they aren't alone. It's a lot easier to fix stuff if it's all out in public. Note, these suggestions are completely different than if you just want to leave a group for your own reasons. In that case, I'd say talking to the DM and then the group saying "Hey guys, this just isn't working out for me right now, because the setting just isn't my bag/I have too many other commitments/etc. Let's keep in touch."