KTM33, the acerbis cap has a rubber insert inside. I have had problems with it sticking to the tank and pulling away just a little bit and not letting the cap seat fully. It seems to happen on cold mornings.
Pull the cap off and look up inside it. The rubber piece should be up in the cap uniformly all the way around. If it is not, then push it up with a screwdriver or something to seat it all the way around. Also inspect the threads. If you have cross-threaded it, then go ahead and get a new cap and start over.
Next, put a little bit of grease, just a thin layer, on the horizontal mating surface on the tank spout. Very carefully thread the cap on, making sure the start of the thread on the cap matches that of the tank. The cap should spin freely and not be cross-threaded. now snug it up some, back it off, snug it up all the way, and shake the bike checking for leaks. It should be fine now.
The grease should help keep the rubber insert from twisting as you snug up the cap and let the tank seat it all the way in for sure with no binding.