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.

It works anyway if you check your subscriptions list, it’s just a graphical bug

I have a number of communitys that are stuck in pending mode for days. I have tried re subscribing with no luck