I have never had that problem. My Marans get bigger than all the other chicks quick. I feel that marans need more protein than normal breeds of chickens. All my chickens actually get higher protein, but Marans in general need more. Try upping it with some eggs or something. I up mine with spent barley (very high in protein) at about that same age they are still on starter. So really I am no help.... but give them some eggs.
I know this was from a while ago but I want to know if higher proteins are ever bad for chickens like they are for water fowl? The littles here are on a grower/crumble but its a lower protein to accommodate the ducks and geese here. I will supplement if I should. I know they get 'meaty' so it would make sense to supplement. Spelt? Where does that come from? I have safflower here.