there a few things that will cause that.
one,fuel over time has swelled the plastic or bonded it to the tank fitting.
two,there is crap under the metal retainer preventing from releasing when you depress it.
three,fuel has degraded the O-rings and they are stuck on the brass nipple unless its made of plastic also,see number one if thats the case.
four,corrosion on the brass nipple if used has increased its diameter making it stick in the fitting.
you may have to heat it with a hair dryer to get it to release,just make sure fuel is not sprayed back into the hair dryer and ignites.
it may end up taking a lot of fiddling with it to get it to release.
worst case scenario is to replace the tank top which should be available through serria marine,boats usa,or any marine outlet if you can find that tank style.