Well, some of them just don’t have enough population to get much activity. In some cases, it might be that nobody has made a community, or that people who are interested haven’t found the community.

If you aren’t already doing it, I’d use lemmyverse.net. When you search on a lemmy instance for a community, it only searches through the communities that that lemmy instance knows about. Instances without a large user population tend to have a limited view of the Threadiverse. The lemmyverse.net guys have a bot that keeps re-indexing the whole Threadiverse.

Your home instance is…hey, another lemmy.today user! So, lemmy.today is a small instance, and it could easily not know about other communities out there.

If you’re not already, hit:

https://lemmyverse.net/communities

Can click to copy the community name (e.g. !fishing@lemmy.ca). Then search for that in the Lemmy Web UI. It may tell you that your instance doesn’t know about it, and ask if you want to trigger a search. Click “yes” and your home instance (in this case, lemmy.today) will go to talk to the remote one (in this case, lemmy.ca), and pick up information about the community.

Once — if — you subscribe to a remote community, your home instance will start to get posts from it. Have to have at least one user on your home instance subscribed for that to happen.

EDIT: If you are subscribed to a community that doesn’t have activity, posting helps – a lot of people are willing to comment, but won’t go out and submit posts. And there’s a community used to help promote communities, !communitypromo@lemmy.ca, which is aimed at helping encourage people to find communities, if you think that you’ve got a good one and want to encourage other people to head over there.