When you target a mob and click obey, your pet will switch targets and attack that mob, so in sense obey has 2 features, places pet in obey if you don't have a target, and forces it to attack the target your focused on while placing it in obey if it wasn't already.
I have Obey in my hotbar and whenever I want my pet to attack a specific target, i click on them and use obey, I don't think I have noticed any issues doing it that way.
Maybe your having issues because your not targeting the mob and clicking obey?