The only times I've had chicks constantly cry is when they have a poop issue. It's very uncomfortable trying to poo when they're blocked up.
Sometimes, pasty butt is not obvious. It could be an internal blockage, or constipation from eating more than drinking. I once had a chick who had something wrong internally and developed vent gleet at a very early age.
I'd check around the vent to make sure it is in fact clean, and look for any bulging outwards of the area, or signs of straining when they're trying to poo.
Be gentle about it and make sure you handle the chicks with support underneath their feet at all times.
If it's not poo related, or temperature, food, or water, then some chicks are just whiny. Those tend to get even more fussy when it's time to wean off the heat light and they have to go to sleep in the dark. Keep supplying the basics as best you can and hope it outgrows it.