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Sometimes you get a hassle with those, where there seems to be a flat shoulder on the shifter boss; it'll probably just be the remains of old shifters jammed on there. Grab and wiggle with vise grips.
Or maybe you have a fat downtube and the radius is nowhere near the same... In the latter case, I've seen them just reefed on so they've bitten into the tube, yuck. File the adapters to suit.
Hopefully an easy question for someone, are Shimano downtube cable stops compatible with a Campagnolo groupset?