Once you have attacked the Rival as many times as they have attacked you, you can not attack them anymore.
If a low level attacks you and defeats you. You can always hit back from 'Rival List'.
I agree that if a lower level attacks you and wins, you should be able to fight back. This level protection scheme is far too restircitve.
You can't attack if the RIVAL is LEVEL 3 or less... above that there is no level check in RIVAL attack backs,.