Hi! I never really play heals, but I tried some days ago and played my diszi priest in a party of 3. I set my role to heal and tried combatbot and Tyrael. I wasn`t in an instance, we were just grinding mobs in pandaria, but my char wouldn`t heal my groupmates. It did damage to the mobs, once i target them, but thats about it. I had to do all the healing manually, which is kind of crappy since I didn`t eaven have skills in my bar :-D I am pretty sure I just forgot to activate healing or something. Maybe someone can give me a hint. It was a plain HB installation, just added Tyrael and using default singular.
Not sure since i dont play priest, but there should be a setting in the class config. Otherwise try a different combat routine.