Hi I dont know if it happens with all heallng spells but when I use redemption almost half of the time it doesnt heal anyone not even me, and i can heal people to full hp so its not about my spell power.
Put harmony mythic into a permafrost hat with a permafrost bonus. Get a legend permafrost hat with a permafrost bonus. Using crystals you can remove everything else from the hat and add 4 empty sockets. Put in the mythstones required for harmony (see codex). Add any other stat using chrystals. All the healing you will ever need. And all the MP too 
I don’t think there is Redemption in Dungeon Quest. you might be asking for support help in the wrong game Forum.