A stuck zipper only gets worse when you yank it. Most stuck things do.

Force applied to a jam usually breaks the thing you were trying to open. The zip is not stuck because you were not pulling hard enough, it is stuck because something got caught, and more force only pushes the fabric further in. The fix is a pause and a look.

Next time something will not move, take your hand off it. The argument, the sentence that will not come out, the file that will not open. Find what is caught first, because until you do, every pull is making the repair longer.