Who knows Bill!
What you could do is to open the throttle fully (by hand) whilst the throttle lever is pushed fully forward (simulating WOT), you could visibly see what sort of gap you have between the roller and cam and this might be some sort of indication of what is going on. If you check out all the linkage and can't see any problems, one solution might be to replace this rubber roller with a hard plastic on (Delrin or nylon 66 is suitable I think), but you will need to have this custom made for you.
Looks Slow but is Fast
Member of the ebay Blue RIB cover club.