It generally seems to be that no matter the club size, 20% of the membership do the lions share of the work to keep the club going. Acknowledge the folks who do, do what you can, and plan events that reward those who participate.Those percentages sound about correct - if not high, for brewing and non-brewing activities I'm involved in.
We have around 40 members. 20 show for meetings and about 4-5 people volunteer to do stuff when asked.
As for getting people to rejoin - most people react to reward the best. Plan something early in the year that members must be renewed to participate in - like a spring BBQ or brewery tour. Punishments, like threatening to drop status, doesn't work as well. And if it does, it only works if members care about the threat. So think about it from their perspective - What is the reward for renewing? If there is no reward, then it's always something that they can do "later".