When i was upgrading my astra bow everytime the game freezed and i was hiting the anvil the enchance failed, my bow went from +14 3 potential to +7 0 potential, i wasted like 20 golden anvils.(also game freezed when i tried +15 , it could have failed too but its something that i will never know)
Few days ago i was upgrading my kaulas leather armor and same happened freeze = fail even at +4 , at the end i got +8 but with 4/11 potential then i tried using multi hit skills instead of AA for the rest of the set. this are the result: Kaulas Gloves +8 potential 13/14 , Kaulas Pants +8 potential 9/9 , Kaulas Boots +8 potential 10/12.
Also if u see the video the second time she hit the anvil it still have 2 more hits so the game freezed and that AA counted as 2 instead of 1 , if that happen it will fail doesnt matter what, same if the anvil was full HP the AA will count as 3 and it will fail, thats why u need multi hit skills to avoid this.