slight grinding while cocking

[CHUCKMATLOCK]

I have a c-fx that Charlie just put a new spring in and replaced I don't remember the name of  a small plastic thing. (cocking shoe?) anyway while cocking occasionally I feel a slight grind about half way through the cocking stroke. Any ideas?

take the stock off and look to see if anything is out of place like the shoe is cracked or missing, the rollers are distorted,look to see if the rear guide is ok.