There are 3 basic type of gas caps:

"NOT VENTED" - As the name implies. Seals tight. Used on cars with another method of venting the tank.

"VENTED" - basically, open to the atmosphere - a pinhole, etc. I suppose this could somehow get clogged or plugged, but this is hard to fathom.

"PRESSURE-VACUUM" - This type has a small check valve built in, allows unrestricted flow of make-up air in; vents to the atmosphere at a predetermined pressure point.

I have found that this last type covers a multitude of sins (venting problems, raw fuel spillage, etc.)

Rick E.