Onix pokemon problema?

Now I have pokemon yellow, and I traded with this kid a Bellsprout for an Onix. I trained it up to 11 to battle Falkner but then suddenly he started to ignore me, etc. Then, without him, I won the battle against Falkner and at first the Onix named "Rocky" didn't pay attention to me but then he started to listen up until now and I don't know whats wrong!