Not certain about making rope, but I have pulled outer cedar bark off to make fire tinder. I would roll it in my hands and that would separate the lose stuff leaving the string like fibers. Those are like a birds nest bundle and can then be twisted into cordage. The fibers are not long but long enough to be twisted.
Get a piece of sisal cordage and untwist it looking at the fibers. Very similar to cedar. Take 10-15 lengths of cedar strings and twist them together to make a rope.
This may be what you are doing already, just adding this to help out...Good Luck