Stop pulling hard on the zipper
Check for fabric caught in the teeth
Gently back the zipper up a little
Use a pencil, soap, or wax on the teeth
Wiggle the zipper pull slowly forward and back
Hold the fabric near the jam to reduce tension
Use tweezers to free any trapped material
Straighten bent zipper teeth with pliers
Clean dirt or debris from the zipper
Lubricate the zipper with a small amount of zipper lubricant
Replace a broken zipper pull if needed
Replace the zipper if the teeth are damaged beyond repair