Unable to respawn Dragon

We killed the dragon back in 1.7 so this is an old world. We just recently tried to respawn the dragon and it did not work. I saw this was an issue with 1.10 but it claims to have been fixed.

I looked into the NBT of level.dat and i have 20 entries in the <gateways> but we never respawned the dragon. the other 2 entries under Dragonfight are DragonKilled: 1 and PreviouslyKilled:1.