Chickens won't let chicks into run

[Leslie2012]

We've had 3 australorp hens for about 2 years and recently added 6 seven week old pullets (3 red sex link, 3 tetra tint)to the coop. (We got them as 1 day old and introduced them gradually.) The problem is that they have been in the same coop for about a week and a half and the big girls won't let the chicks into the yard. Everytime the chicks all spill out of the hen house into the run, a hen will run them right back in. It's getting into the mid 80's during the day and the hen house is getting hot- the chicks need to be able to go outside! I'd appreciate any idea's on what to do!