A review for Rectangle stickers by Blaine Sundrud.
UPDATE EDIT: StickerMule customer support contacted me immediately on my response here and reprinted the entire run. New stickers came that week and had the gradient I needed. Absolutely worth changing the review! (End Update)
TL/DR disclosure: If I had looked close at the proof I would have seen this - so... my fault here.
I built a sticker for dev laptops in InDesign with a Gradient on the text layer (orange fading to black). When I got the stickers the gradient had been flattened to greyscale only although the shadow work was still visible. When I re-checked the proof to see if the error was in the printing, I saw the proof had the gradient missing as well.
The quality of the stickers are great and we are still using them, but they lack some of the original design intent.
So the lesson from all this is, make sure you examine the colors in the proof screen, not just the physical elements