I had a DeGraaf trailer and my rollers started falling off.
As you assemble them on the shaft, it goes, large mild steel washer, plastic roller, large mild steel washer, mild steel split pin.
The common theme here is "mild steel".
It rusts like buggery in salt water.
I replaced all the washers, if I could have got hold stainless ones I would have used them, and replaced all the split pins with stainless ones, (no more lost rollers).
It baffles me why trailer manufacturers do not do this from scratch, assuming they are not making a living from selling replacement rollers.