Ok - I have done thousands of caps - literally. I use a Xyron as well. However, I sand the tops of all my caps before I adhere the image. I fold up a towel, but a piece of sandpaper on it, and quickly scrape 2 caps at a time on it - put them to the side and then do more. It goes very quickly. I adhere my image, and then flip it upside down and press it HARD on the leftover white paper that I peeled the sticker from while that is laying on a towel. Never had problems with lifting.
That said - I did find issues with some caps I had done a year before that I had stored in my basement during the summer. Too humid or damp and all of the images lifted. Now I make sure to store them in a dry space upstairs. HTH