It's almost certainly the material, not sure why it would be fine one day and not the next. Maybe the banners yesterday were lighter and it wasn't as noticeable?
Cleaning the surface before printing with isopropyl alcohol will clear it up, but it's a huge pain. If it's newer stock (to me) I send it back, but if it's been sitting around for 8 or 9 months I'll just toss it, not worth the hassle for how cheap banner is.
If you search the HP forum for "wave" or something similar there are lots of posts of people having this problem.