I noticed this too, I worked around it by:
- Going to the community
- Clicking on "pending" (this seemingly cancels the subscribe request)
- Selecting "subscribe" again
It seems hit or miss but that made it work in some cases, a few others I left for a couple of minutes then checked back on my subscribed communities - https://feddit.uk/communities/listing_type/Subscribed/page/1 - to see the subscription had become successful.
I largely suspect this might be a load issue or something with federation. I imagine there's a lot of stress on various parts of the Lemmy federated infrastructure from the influx of reddit users.