When I was trying to defeat 3 Withers at once, I got this problem.
1. The Withers were destroying the packed ice in the river so I kept falling and couldn't get up easily.
2. The Wither was destroying the blocks quickly enough so I couldn't get back up, but noticed that the stone blocks haven't been destroyed.
3. The Wither was higher than how much further I can get from the ground so I couldn't do melee attacks from the ground.
4. The Wither is immune to fireworks and on 2nd phase the Wither boss is immune to arrows and tridents so I have to hit it with melee that time.

With issues number 3 and 4, I recommend using an Elytra to be able to hit it easily. I was fighting them in the overworld. I was using full Netherite armor with a stack of enchanted golden apples as my food source and a Netherite ingot with sharpness enchantment level 101 as my weapon. While I was trying to fight the Withers, I noticed that I have another Netherite boots. I was not sure if it was a bug that I somehow got an extra Netherite boots or if I accidentally copied the boots while trying to equip them.