1 did you pick them up or have them shipped from a hatchery? if they were shipped, it is possible they got chilled or shaken up...
2 could you take a picture of their brooder/living area? what kind of bedding are they on? cedar shavings/newspaper could kill them... try pine shavings instead.... do they have enough space? are they getting trampled by each other? is the brooder predator proof? could rats/mice/snakes/etc. be getting in and killing chicks?
3 how warm is it in the brooder? if it is too hot, that could be a problem... if it is too cold, they could be piling on top of each other to keep warm and getting smothered.
4. is the water fresh? did you put anything in it? sometimes too many electrolytes or other things can kill chicks..
5. is the food fresh? does it smell fresh? if it is moldy, that will kill chicks... have you been feeding them anything other than chick starter?
6. who handles the chicks? just you? any children?