I'm not trying to act like I know a lot or anything, but the one thing that I have learned pretty much by myself in the last 6 months, and it seems that others have not made a lot of constructive informative comments regarding this subject, is that new rope behaves much differently than well used rope.
I have left my ropes out in the weather, rain specifically, and washed them many times to try and get the production waxes off of them. I think abrasion and/or uv light exposure are necessary. UV is in short supply in the winter though.
Anyway, what I'm trying to say is that my ropes act completely different until they have been well worn and exposed. Slippage is common until I have gotten the production waxes off the fibers of the rope. And it takes a lot of work to get these waxes off the rope. 250lbs, and my rope runner (wide open setting) won't hold without slipping on any rope until that rope is "conditioned ".
I hope that isn't opening a can of worms, much more could be said about the subject.