Respawning Ender Dragon doesn't work after 1.10 update

A world in Minecraft Realms created in 1.9: I've never respawned
The Ender Dragon before. When creating a copy of the map using the same seed still works. I've only spawned 2 withers in the end end hollowed out an obsidian pilar so I can farm Endermen. After pla
cing the crystals nothing happened. Could it be because of the 1.10 update? This map on Realms has being worked on very long. I'm the worst in editing files or so, so please find a way I can still respawn the Ender Dragon.