If all the chicks are doing that, and their parents did as well, then it certainly sounds like you have them pure for the mottling gene.
Maybe find a mottled chicken that shows excessive amounts of white (some other breed), and mix it in? That might bring in whatever genes allow them to express the mottling more strongly as adults.
Or maybe just keep selecting for the ones with the most mottling (not enough, but less bad than the rest), which is what I think you are doing.
If they have varied amounts of mottling, have you already watched whether there is any correlation between how much mottling they show when young, and how much is left later? It seems like the ones with more mottling when young should be the ones that will show it more later, but I've seen some chickens from mottled breeds that show little or no mottling when young but have plenty as adults, so maybe it's something that works backwards of what would seem obvious? (Or maybe it's not. I don't know for sure.)
