Villagers are despawning everyday

Some villagers in my minecraft village despawn whenever im not in the village. Sometimes i go to my home and sleep, come the next day to realize i have a missing villager. Now i have seen that some villagers leave their doors open when they sleep, and it may be a zombie who kills them. But my village is pretty well secure with over 6 iron golems. I believe it is a serious issue since some days i seem to lose villagers that are vital to my villages improvement.