The first time I fought the ender dragon it would take damage until it got down to about 75% health then nothing would affect it, neither arrows or sword but the dragon can still cause me damage. After that I’ve returned three times and none of those times would the dragon take any damage, not even turning red to show I hit it with an arrow. 

I broke all the crystals and I’m shooting it in mid air when it’s not on it’s perch or hitting it with my sword when it flies by me and nothing will cause it damage. I’m tired of using diamonds to make my weapons and armor and enchanting them only to lose it all because the dragon won’t take any damage. Is there any solution for this or am I wasting my time?

Playing on Xbox one version 1.12.1