Then, ensure you’re making space for them to get the thing they’re showing up for. Show a direct connection to how what they’re doing supports the outcome they’re trying to achieve. Show clearly that without them performing this task, the outcome they’re invested in will be compromised.
This game is all carrot, no stick. The only threat you have is that you might forbid them from continuing to counter their time, and that’s usually gated behind a pretty laborious process.
If someone’s primary reason is something like “I want to protect my community” then you can probably rely on them to keep their nose to the grindstone on less-fun tasks. If their driver is “I like having coffee with other other volunteers in between working” then they’re still an asset, but you probably need to hand them short simple tasks.
Expect attrition in the group and always ensure you’ve got new blood coming in. Be prepared for tasks to get dropped on the floor if someone gets busy or bored. If something absolutely must happen no matter what, that’s a job for a paid member of staff or it needs to be distributed amongst a large group with a sophisticated collaborative structure.
Source: volunteer firefighter that manages an equipment maintenance team and spends way too much of his time doing stuff because he loves it.