I have the same machine as UCMJ22 (Roland XC-540). It only seems to happen to me when running banner material. I only use the two outside pinch rollers, so that the middle ones don't leave tracks. I also use the media clamps to hold the edges down. But it still seems to buckle on the edges, especially if I have a new, heavy roll of banner on there. I do not have a problem with the take up reel, I just make sure to tighten the screw pretty tight.
Two days ago I was running a banner with a full roll on the back. I roll out some extra on the back side and let it hang, so that the rollers are not trying to turn such a heavy roll. Then I just try to watch and run some more out on the back when needed, it sucks because you have to watch it every so often. But I can see the printer from my desk. I still have to pull on the banner on the front until it gets down to the take up reel so that it does not buckle. Then I figured if I hung some clamps on the edge of the banner, the weight would keep the banner from buckling. It has worked so far, I just take the clamps off when it gets down to the take up reel, and everything is fine from there.
It is a little bit of a pain in the butt, but it only happens on banner material.