Question

Do you have to keep fishing on the square when it says not even a nibble or do you just move on when it says that

Answer 1

Well, first off, you have to make sure it's the right spot for the Feebas to be. Then, try a few times more, and if nothing happens, then move on.

Hope this helps =)

Answer 2

Try once or twice more just in case you where un-lucky.
This way you can be sure.