I'm introducing (2) three month old hens to a older hen and a 4 year old hen. Our 4 year old hen will not let the younger hens into the roost at night. During the daytime its OK they just avoid each other but the nighttime is a problem. I put the younger hens in the roost last night but tonight again our four year old hen will not let them up in the roost. Any recommendation on what I should do?