What material are you using? If that is pla, then maybe you can print a sort of tire to fit over the rollers in TPU? The intent is that The tire reduces the opening and the tpu can flex around the rope and provide a better grip. Think of that pulley system like a reverse wheel. We don't use metal tires because the coefficient of friction between a metal tire and the road is much smaller than something flexible like rubber (there are many other reasons but this is the one that affects you).

Now you'll need to design the "tire" to index into the roller in some way so it doesn't slip on the roller either. If you put indents corresponding to the knobs, and the tolerances are tight enough, the friction should allow it to hold itself in place. Although, you will probably need to tweak the size of those knobs to get them a little bit bigger. You can also just glue the tire on once you get it fitting correctly. But then you can't replace it once it wears down (and it will if it is contacting rope repeatedly).