Digital BW, The Print

I made a print using 11 x 17 Hahnemuhle Fine Art Pearl only discover to 
what I can only assume are faint "pizza wheels" running top to bottom 
of the print in the pure black areas. I made another print using Museo 
Silver Rag and it turned out fine or at least the vertical banding 
isn't noticeable to my eyes.

First, can someone explain why this would happen on one paper and not 
the other? Secondly, what can be done to eliminate this problem without 
removing the roller wheels.