That all chicks can be feather sexed is a common myth on here. For chicks to be feather sexed they have to be set up correct genetically. The mother has to have the dominant slow-feathering gene and the father has to have the recessive fast-feathering gene. The mother gives he dominant slow-feathering gene to her sons but her girls get nothing from her. The father gives a recessive fast-feathering gene to all his kids. So the boys feather out slowly since that dominant slow gene controls but the girls feather out faster since all they got was their fathers fast gene.
This is not a breed thing. Once you make this cross the genes are mixed up so they are not set up correctly. Some breeds are auto-sexing but that has to do with down color, not fast or slow feather growth.
I don't know what breeds or colors the parents of yours are. If they are different it's possible that we can help if we know that but it is important to know which color/pattern is the father and which is the mother.
Many chicks start showing differences in sexual traits at 5 to 6 weeks. If you post photos then we might be able to help you. For each chick show a head shot so we can see the comb and wattles if they have any. A profile shot showing legs and general posture can be a big help. And tell us how old they are.