Named sheep despawned

Hello, 

I have been playing daily in my Minecraft world on Realms. I had about 10 sheep of various rainbow colors - 3 of which had name tags. I made sure to check on them every day (in game) and that they had plenty of space. Today when I logged in I found that many have despawned - including all three who were wearing name tags. 

Any advice? I'm obviously frustrated. 

Thank you, 

 

Darcie