#Guppe for us has been a problem for a long time. Users on our Firefish server just can't follow them. #Chirp too has issues. Especially inbound. The responses are malformed. Right now I have 11 in our Inbound queue which will eventually fail and disappear, so the subscribe user will never see the message.
From what others more knowledgeable that me have said, the implementation of ActivityPub for these services is poorly constructed and have been cobbled together for Mastodon servers