enemy set to be prefab

I have make one of enemy and an object that enemy attack it. The setting is finished. When I put this enemy gameobject into prefab so that it can spawn out by time. The attack target have missing and I try to put the target back. It say the target need a gameobject prefab too. So I make a target prefab also. Now the problem is when they are not prefab. The health bar can detect the attack and move on display. If they are prefab, the health bar is not work but I can see the inspector preview the health is decreasing. What is the problem occur? Please help.