Yeah it can be a bit odd, Amethyst in particular doesn't always post to all relays, especially if your current Internet connection isn't great. Some relays are paid, etc.

nos.lol is the biggest free one I think, nostr.mom is pretty high up there too, I run n.ok0.org, it's not large but it's free and takes notes

If you go to relays Amethyst will show you if the relays are paid relays or not, paid relays won't accept notes until you pay them.