What you are describing doesn’t sound all that strange to me with them at that age. Are you familiar with a regular mating ritual? The rooster dances to signal his intentions, the hen squats to get her body on the ground so his weight does not go through her legs, the rooster hops on and grabs the back of her head (That head grab is her signal to raise her tail out of the way so he can hit the target), he touches vents and hops off, then the hen stands up, fluffs up, and shakes to get the sperm in the right place.
There are a lot of possible variations of this that involve her trying to run away and him chasing, but as long as she squats to spread his weight, it generally works out pretty well. Occasionally you get a real brute of a rooster that just never learns proper technique or how to be a desirable mate. Occasionally you get a hen that just won’t submit. These can cause problems. Both have to do their part. It’s not always just the rooster.
I suspect a really big part of your problem is their age. They are still kids, just adolescents. The rooster is running on full hormone load. He probably hasn’t matured enough to learn to control those urges and very possibly doesn’t have the proper technique down. Although they may be laying, those pullets may not have matured enough to do their part, so they resist.
There is more to it than that. A good dominant rooster has a lot of flock responsibilities. He not only should dance for the hens instead of just hop on, he needs to find them food and let them have the good bits first. He needs to break up fights and keep peace in his flock. He needs to watch for danger and give them warnings. Often a good rooster will help a broody take care of her chicks. He needs to take care of his flock. He also needs to dominate the hens just with the force of his magnificence. I’ve had a 4 month old cockerel that could do that, but that is extremely rare. It usually takes longer, maybe until they are a full year old to get it right, though most are faster than that. Some hens will squat for anything in spurs but many, especially when they mature a bit, expect a rooster to earn their respect before they submit.
In a flock with an old mature dominant rooster and older mature hens, the dominant rooster will let the young boys know they are not at the top of the heap. Sometimes they run him ragged keeping peace in his flock. Mature hens will often school a young cockerel in the proper ways to treat a lady by smacking him around when he gets fresh. It’s still a frenzied time when those young boys are growing up, but your flock doesn’t have that advantage of older chickens to help teach the boys. Yours just have to go by instinct alone, the cockerel and the pullets.
I know this is long and rambling, but there is more. The mating ritual is not just about sex. It’s also about dominance. The one on bottom is accepting the dominance of the one on top, whether willingly or by force. For a flock master to do his duties, he has to be the dominant chicken. How can he break up fights if the others might just beat the crap out of him? While a good part of the mating is sexual, a good part of it is just him establishing his dominance over the flock.
It’s also very normal for a rooster to mate a few hens first thing in the morning, even a mature rooster. It’s to show them that he is still the dominant one as much as it is sexual. The same thing will happen if you take him away from the flock for a while then put him back. He will normally immediately mate with a hen to show he is still he boss. Or if you take a hen away for a bit and put her back, he’ll often welcome her back by mating her.
It is always possible you do have a problem, but I think it is something they will grow out of as the cockerel and the pullets mature. It can be rough for you watching them if you can’t accept chickens being chickens. I’ll mention the only real reason you need a rooster is if you want fertile eggs. Any other reason to have a rooster is really just personal preference.
Good luck!
