its not the printer, its your grubby hands leaving oils on the vinyl that won't properly accept the ink being laid-down.
i printed on 5600 for years with my mimaki JV3 and thousands of parking signs, as well as my service trucks, still have beautiful printed reflective graphics on them to this day.
if you're trying to print on materials that may be contaminated, wipe it down with a lint-free cloth & isopropyl alcohol prior to printing. if you're buying reflective vinyl by the yard, and not full-rolls, then assume that it has been handled/contaminated.
edited: your banding IS, however, a printer issue. can't say exactly what, but by the variance in colors I'd say you have printhead/ink flow as well as alignment issues, and I'd bet you don't have ink limits setup properly for a reflective material profile.