My tank cap and lock had been working fine on the 1975 DT250B I was restoring to service. 
But this morning, after my third effort of shaking the tank with a large handful on roofing nails and a couple ounces of alcohol inside for rust removal, I was unable to remove the gas cap.
The cap shows no appearance of "cross-threading" or being crooked.  When I turn the cap about 45 degrees, it just hits a hard stop like the tabs are bumping into firm metal.
To me, it feels like the cap is not unlocking but the cap was not locked when I started the latest shaking effort.  And I think I've determined that the cap did not somehow become locked.  When I intentionally lock the cap, it only turns about 15 degrees (rather than 45) before stopping.
When I pry up on the lid, the spring in the mechanism allows the lid to lift on the side I pry.
Did a nail somehow get lodged under a tab and prevent opening?
Is the solution just prying hard enough to bend one of the tabs until the lid releases?
Thanks for your help with this matter!!

Sounds like something got wedged to me.  

Another thing to watch out for is drywall screws or nails can get stuck in the tank itself.  I read an article once that said to count what you put in and then count what you get out after.  I did and found that several pieces did not come out right away.  I couldn't see them but obviously they were stuck in corners or something.  Took quite a while to get everything out of the tank.  That was the first and last time I used that method!  LOL

It sure sounds like one of the nails got stuck up in there. Most people use nuts and bolts, and even short pieces of small chain. However what really works well is Evaporust. Get a gallon and pour it in, then let the tank sit the bottom, side, top and other side each for several hours each. Then rinse it out good, and mist it with some WD-40. Try to avoid letting Evaporust stay on the outside paint, and rinse it down with fresh water, though I've never seen it do any damage. I think if your paint has a lot of metal in it it could cause an issue.

Thanks for your suggestions.
I did probe the lock with a thin metal spine from a windshield wiper blade to no avail.
I was able to pry the cap out of the filler hole while using a rubber pad to avoid damaging the tank. I could find no blockage or other reason for the cap's refusal to twist the cap far enough for removal.
The Delrin/Bakelite plastic body of the lid cracked during removal but I was able to heal the crack with Loctite Professional Super Glue.
The cap is back in service and just slightly worse for the experience.

Man puts nuts in his tank, motorcycle is visibly upset, man goes nuts !

(old bike chain does seem to be a safer option) Can you poke it with a long thin piece of metal going through the petcock hole ??? Just a suggestion