Make sure you pulled off the right side. The decals are papered on both sides. One side reveals the sticky surface, you stick it on then carefully peel off the other side.
I put one on my samco intake duct. They really aren't very sticky, but seem to stay on fine once there. Make sure the surface is clean, though, or the minimal stick will get used up on the dust.

(went back to the stock duct with Haibeck inserts though)