Part of the mating ritual is that the rooster grabs the back of the hen’s head. That’s her signal to raise her tail up out of the way so he can hit the target. If that doesn’t happen, they don’t mate.
How old are they? It’s fairly common for a cockerel and a pullet to not have very good technique. The cockerel’s technique is more important but they both have a role to play. A hen losing a few feathers either on the back of her head or her back is pretty common and is not a cause for concern, but if those areas start to get bare it becomes a concern. You don’t want the hen to start bleeding from an injury. It’s possible the other chickens will become cannibals.
If the feathers are totally removed they will soon grow back. If just a tiny bit of the shaft is left in there, that feather will not be replaced until the next molt.
Normally if they are still fairly young they will mature out of that phase and learn better technique. I can’t tell you how long that takes or how much risk the hen is in. I see that sometimes. I even see it on older hens, but it is usually just one of the older hens which makes me think it is something about her more than the rooster. If it were several hens I’d think it more likely the rooster had a bigger part to play in it. I’ve never had a hen get injured from that and some have been pretty bare on the back of the neck, but others on this forum have had hens injured pretty badly that way.
The way I handle it is that if there is no bleeding or injury, I leave them alone. When I see bleeding I isolate that chicken until they heal up. The only time I’ve had to do that was when a hen was pecking the feathers off a rooster’s throat and he started bleeding.
