DIAGNOSIS This is usually because there's still a vacuum (negative pressure) inside the canner. The pin is supposed to drop when pressure is equalized, signaling it's safe to unlock. If it's stuck up, pressure isn't equal.
FIX Don't force it. Just wait longer. Sometimes it can take an extra 10-30 minutes after the gauge hits zero for everything to fully equalize, especially with a large, full load. You can also very gently nudge the weighted regulator on the vent pipe to see if a tiny bit of air hisses in or out—if air whispers in, that confirms a vacuum. Let it sit until the pin freely drops.
IF THAT DIDN'T WORK The pin itself might be sticking due to mineral deposits. Once the lid is finally off and cool, lift the pin and work it up and down. Clean it and its housing with vinegar or a descaling solution to remove any hard water buildup. A drop of food-safe mineral oil on the shaft can help it move freely.
HEADS UP Never try to cool the canner rapidly by running cold water over it to force this pin down—that can warp the aluminum lid or base. Also, never use tools to pry the lid open while the pin is up; you'll damage the locking mechanism. Patience is the only safe tool here.