Nab healers dont use Avatar for heal cause it's not cost efficient (530 mana) and it has a big cooldown (1:30) full buffs. In the same ammount of mana and 1/30 of the time u can cast 2,5 chain heals that will heal more than 75% hp than avatar that will only heal 35%.
Now about your issue the skill works just fine and your skillgrp is just wrong. Remove Improved condition and the Body of Avatar will heal you just fine.
The problem is when you have improved Condition and use BoA buff system recognises it as inferior skill and doesnt give you the effect at all cause Condition give you 35% HP and 35% MP. That's how l2 was designed to work in off/off like server.