They should start clucking eventually. Although my Barred Rocks are just generally very quiet, whereas Sussex are a little more vocal (from my experience).
I have about 7 4-month old crosses with Barred Rock and EE blood. They still peep. They'll grow into clucking eventually though.
My BR's didn't start "clucking" until POL. But they are still VERY quiet in comparison to my BO's and Production Red's. Usually the just go "brrr, brrr, brrr" very quietly. The others are "BWAAHH, BWAAHH, BWAAHH" loudly.