91 radiator problem.

If it is the proper cap for the rad, then you have rotated it enough to remove. It sounds like the seal has become glued to the tank-cap-seat. Force will be required and a new cap afterwards.

If it is not the correct cap, then it has become jammed-on mechanically. To remove, you may have to prise the metal-skirt or sides of the cap down then outwards using strong long-nose pliers. This will free the lip from the cam-bumps on the rad-neck. It then will pop-up.

I had this happen to me once. I thought the cap I had was the correct type, and I used more force than usual to close it,....I paid for that piece of stupidity :-)

Jason