Natural Endercrystals are invulnerable

While fighting the ender dragon in my survival world, the endercrystals became invulnerable. I had already destroyed about 4, but now they can't be destroyed. Not by hitting or shooting with arrows. I've seen a few people with this problem but their solution was to go to creative/turn on cheats. I would like to not do that because I want the achievements. Right now my survival world is on pause until this issue gets fixed.