Two things to try:
1. Pre-feed the entire job, then scroll it back. Or, short of that, at least loop the banner in front of the feed roll so the printer is not pulling the full roll when it feeds.
2. Feed out a bit of banner and using butterfly clips on the front of the banner, hang modest weights (or a broom stick) from the leading edge, which will pull the media tight across the print platten.
Good luck, let us know how you resolve the problem.