Minecraft sheep aren't growing their wool back

Sheep aren't growing wool back in the Minecraft server I play with my friend, before i updated the game it was still working. Didn't test it out yet for the single player game though

What I expected to happen was...:
the sheep would grow the wool back

What actually happened was...:
they didn't grow their wool back

Steps to Reproduce:
1. Sheered 4 sheep
2. 5 nights later they still didn't grow their wool back