My guess would be cause Pet_HP is one of those Hard Coded bars. It's set to automatically turn off when you don't have a pet when it's within the PlayerWindow.
Make a pet and I bet you'll see it.
Try putting the Pet_HP EQType back, and then using another bar. Try and just copy oh, the Mana Bar (copy - Change or just Remove the ScreenID) and change the EQType. That should work.