Assuming there's no obvious visible outer problem with the filler neck, then it's possibly some foreign object has somehow found its way in to the neck...ie maybe the little flap that the fuel filler nozzle pushes down, maybe one of these collapsed and went down the neck and was replaced without fishing the now disappeared one out.
Maybe a peruse on the three make's specific forums might reveal a reason, someone there will have had to remove/refit the filler neck and hopefully posted a thread about how to do it.
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