Despawning Issues For Months

For months and months, my server has been affected by mobs despawning. Including mobs that supposedly don't despawn (i.e. iron golems, villagers) Nametags don't prevent it from occurring. It's affected almost every single mob in the game minus the bosses. It's been occurring for a long time and is getting to the point where I don't even want to leave the general areas of the mobs in fear that they will despawn if I'm gone too long.