When we introduced 2 Jersey Giant pullets (about eight weeks at the time) to our flock of 8 two year old Australorps, there was bullying. Was a few days before they would emerge on their own from the coop, and we spent the better part of the summer with the pullets in the run while the flock was...
Could be that they are still smaller and being bullied. Chickens usually sort out their own politics and as long as blood isn't being shed and no one is being injured it should be okay in time. They'll grow, gain confidence, and find their place. In the mean time, while everyone else is out...