They sent riser shelves (14", not 18") last year. It's not something they would resend every year as that size isn't used too often. We kept many of ours on the gondolas since fixture room space is almost always non-existent. The rest we made for into the fixture room.
I've also heard there is a supplier issue with a lot of fixtures. Ordering shelves isn't going to do any good if the supplier can't ship them.
We normally end up with a shelf shortage going into the seasonal set after Christmas because it uses so many, but never before. Right now, our fixture room wall is 60% full of shelves still, but come January will be completely empty. We also replaced and use 22" shelves everywhere the basedeck is 22." Seasonal and mini are all 22" for us so that frees up 18" for elsewhere.