Named Mobs keep on despawning

(**Note=skip to second paragraph if u dont want to read my story)

I have been playing on my SMP server for over a month now, it was going great. I decided to build an iron farm (which took me over 5 hours btw just to get a zombie and villager to the required area) and then i put the zombies and villagers in the chamber. After doing that, i decided to leave the game (yes that chunk was near spawn so it was loaded all the time) and after like 2 hours i logged in to see how much iron i have made, and saw that there was no zombie. I remember naming the zombie so that it wont despawn. So i had do to it all over again and after doing the second time, i logged out for 1 min and logged back in. The zombie was gone again, with nothings, no drops, not even anything. It was impossible for the iron golems to kill the zombie, so i was confused. I decided to try to make a different server to test it and found out when i named a zombie and logged out or went too far from it, it always despawned. And it even happened to hostile mobs and not passive mobs.

Can anyone please help me how to stop hostile mobs from despawning? I made an SMP server and also want to make an iron farm, it would be very helpful.

Here are my plugins: