I realised the problem! I was using Sync which doesn't seem to support Piefed. I've just installed Summit and I can now see all the Piefed communities in my search. Thanks for your help!
Addendum: Yes we know that you think ml/hexbear/grad are tankies and or .world are a bunch of liberals but it gets old quickly. Try and come up with new material.
Ah OK, maybe that's it - thank you!
Also, "all" is not all. It's only the communities that others on your instance have subscribed to.
I realised actually that when I search through my federated instances list, not a single piefed instance appears... :/
need to make sure you have selected All when searching. I find piefed instances occasionally, like the TV & movies ones are pretty active I think.
I realised the problem! I was using Sync which doesn't seem to support Piefed. I've just installed Summit and I can now see all the Piefed communities in my search. Thanks for your help!