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.