Did you make the can stretcher by pressing an unopened can into the bottom portion of a similar can? If so did you polish the bottom (stretcher) portion with fine steel wool, to remove the paint and smooth it? If it still sticks you might try spreading a drop of oil, or silicone spray, on the stretcher can before stretching the stove can. I've never had mine stick. That would be a real pain.
Note the small hole in the bottom of the stretcher can, which allows the unopened can to bottom out.
Side view of Stretcher Can
End view of Stretcher Can