Hi, I had similar plastic on mine but when I removed the blue bit I found the metal box section had corroded and so I replaced the whole lot with the things called Snubbers from here
Boat Trailer Rollers-Boat Trailer Parts-Boat Trailer Roller Bunks Delivery took a while, but when they arrived they look okay and are now fitted. I've yet to launch and recover so can't say how well they slide. Scroll down the list on the page. There are also a number of other options for bunks etc.