Stop pulling harder
Check for fabric caught in the zipper teeth
Gently wiggle the zipper pull back and forth
Use a pencil, soap, or wax on the teeth
Apply a small amount of lubricant if needed
Straighten any bent zipper teeth with pliers
Pull the fabric away from the zipper path
Hold the zipper track taut while moving the slider
If the slider is damaged, pinch it slightly with pliers
Replace the zipper if it still will not move