In the experimental versions the axolotls do not spawn because of three reasons that I would just like to make apparent that the spawn conditions for the axolotls are almost never met.

1. The light level in lush caves is usually lit up with glow berries.
2. The ponds of water are often too small for axolotls to spawn.
3. The water isn't very deep and is floored with clay which the axolotls cannot spawn on.

Please note these and change them at your earliest convienience.