I really can't give you any tips on what to do, because I have the very same problem.

I am in charge of a regional search and rescue team, and when there are no searches, people get bored and lose interest. When they've gone through most of the training, they are not interested in monthly meetings. I'm telling you, i'm just about to pull the plug, members don't even care to show up on searches anymore. There are many reasons for this lack of motivation, such as personal issues, political issues, the fact that police do most of the work now and don't call volunteers soon enough, etc.

Sometimes it's just not worth the effort. Just a thought.