here is a ss i took of me trying to follow some random account on nostur with the follow button greyed out. surely not helpful for you but at least u can see 
Discussion
Will tag nostr:npub1n0sturny6w9zn2wwexju3m6asu7zh7jnv2jt2kx6tlmfhs7thq0qnflahe to see if he knows what might have caused it to be greyed out. My guess is a relay issue 🤷♂️
Most of nostr issues are related to relays