a

[ghost]

a

[DigitalWendigo]

It might be open terrain generator.
If you're using a preset, it might conflict with how dragon dens spawn.
Try to reproduce this issue with vanilla terrain generation in a temporary test world.

Another thing I can think of is biomes o plenty. I've personally not used that mod too often, but I have used 'oh the biomes you'll go'. In which case, I needed to add the new biomes from that mod, to the biome spawning conditions of each ice and fire mob inside the ice and fire folder.

[fallenthereaper]

1.12 is no longer supported

[DigitalWendigo]

This can also probably be closed.
No point in keeping issues open on versions that are no longer being supported.

[DigitalWendigo]

As a side comment on the issue before it's closed.
Try adding the cave biome id's to the respective dragon biome.json files found inside config/iceandfire.
Custom biomes need to be added manually to the ice and fire creatures .json files for them to spawn in said biomes.