The two thick horizontal chrome bars are secured by blue castings, one at each end. Your pictures show the left casting. Each casting has two cylindrical portions that bars slide into. Your close-up photo shows the upper cylindrical portion of the left casting, with the mating chrome bar to the right of it. Each cylindrical portion has a screw behind it which secures (holds) the casting on the bar so it won't slip. I suspect that one or both of these screws are loose on your castings, allowing the castings to slip inward, leaving the gap where the nylon wheel slid out in your picture. If so, the solution is to hold the bar with one hand, and with the other hand, slide the casting away from the bar to remove the gap, and tighten the screw. I suggest checking all four screws. Good luck.