If you have a dragon head that is being constantly powered, it will eventually stop animating. The only way to recover the animation is to break and replace the dragon head. Simply disabling and re-enabling the redstone signal does not fix it.

The specific time it takes for this to happen is unclear. When playing via a Bedrock Realm, I see this happen after probably one or two real world days. It does not appear to be related to the chunk it is contained in being loaded/unloaded.