Re: center rolers ?
Most of the weight should be on the bunks. On mine, the taper of the hull shape allows the whole hull to glide on the center rollers until it is nearly all the way to the bow stop. But about 18" or so from the bow stop the aft hull shape glides onto the bunks lifting 95% of the weight off the rollers. So I have mine set up so that some of the load (very little) is on the rollers. I have four rollers from the transom up to the prow.
When I rebuilt my trailer Designed that in: I positioned the boat on the rollers, slightly higher in the front, and then mounted and welded the bunk brackets with the bunks installed but with no carpet.
After all that was worked out, I installed the carpet- viola; the bunks take most of the load when loaded, but the rollers still take up a little weight, and the boat loads very easy. (when I first installed the carpet there was air over the rearmost two rollers. That went away after a few months.)
If you have a black streak I think that your rollers are too high and therefore have flat-spotted due to the weight of the boat being too concentrated on the keel and not enough on the bunks. My rollers leave no marks or mars, and roll very nicely.