When I did mine I applied the decal, then worked as much air/water out from around that ridge as I could and then let it set for a while. Every now and then, for the rest of the day, I would go back, pick the gauge up and work the decal into the ridge. It seems that the air/water would somehow work it's way out maybe through the face of the decal and it would stick better to the contour of the ridge. The next morning I went over it one more time before I installed it and it stuck and has stayed since.
Hope that helps.
