If you search for a certain community and find that yeah it exists but nobody has posted there in 6 months...

I've come across this multiple times, and have had (or seen) some success with the following:

• Start posting to the dormant community. There are usually still some people subscribed, so you'll get a few upvotes.
• Reach out to the moderators. If they don't respond, reach out to the instance administrators to request the community.
• After a few weeks of posting regularly, promote the community in !newcommunities@lemmy.world, and any related communities with more subscribers.

I think the "if you build it they will come" adage applies quite well here.