mine was sticking once at a gas station, the nipple got stuck in the open position and the door wouldnt stay shut after fueling up. I found some 3in1 light oil inside the gas station, bought a bottle and it loosened it right up. few drops an some vigorous pressing on the plunger did the trick.

My release machanism works, but whatever forces it open doesn't. I use my key to pop it open after I release it. What is supposed to be there that makes it spring open?

Its just a peice of metal under that rubber bumper (if you still have it). You can just bend it back out by hand and it should pop out by itself.