Are you using the procedure in the service manual? The one where you put the cord around the seal and then pull on the cord which pulls the seal into the inside? Also, make sure the seal is good and warm. It should be almost too hot to handle. So, you will either want to do this in the sunshine, or with heatlamps trained on the back window. You will need at least one helper on the outside applying gentle pressure while you are pulling the cord on the inside.