Putting four ender crystals in the appropriate locations on the end portal does not respawn the ender dragon, nor does it respawn the crystals on the obsidian towers. The crystals just sit there and do absolutely nothing. I am in normal difficulty and I tried every factor I could to see what was preventing the dragon from respawning. I have seen people make similar issues, but I've only seen it with the clay creeper face from many snapshots ago.