Mobs despwaning

Having a problem in our realm with mobs including villagers and horses despawning even when named. It's impossible to create any farms when mobs are always disappearing as it's hard to get new mobs back into built farms. No-one wants to go looking for cows for ages every time they wanna farm some food just because the cows at the cow farm are all despwaning. Plz fix this despawning issue thanks