The most frequent problem that has to be corrected on a metal zipper is when the sides separate and the teeth won’t join. If there are metal teeth missing it may not be repairable unless the missing teeth are at the very bottom of the zipper. Unlike plastic zippers, metal zippers can be repaired as long as you still have the zipper pull tab. The key is to make sure the tab is as close to the bottom end of the zipper as you can get it. Zipper repairs must be sewn by hand.
Make a small cut in the edge of the zipper between the bottom teeth on the side that has separated from the zipper. The zipper tab should be on the side that is not cut.
Hold the end of the zipper you cut firmly and zip the zipper slide up about 1 1/2 inches. Stabilize it by inserting a small safety pin until you have sewn the two sides together.
Thread the needle with a double strand of thread and knot the end. Use thread that matches the color of the zipper fabric.
Insert the needle into the back of the zipper on either side just slightly above the cut you made, so that the knot does not show from the front. Bring the needle up through the fabric to the top side
Insert the needle on the exact opposite side straight across the zipper to the other side. Do this seven or eight times so that the two sides of the zipper are firmly connected, but laying flat.
Tie the thread off on the back of the zipper. Remove the safety pin if you used one.The zipper tab should work smoothly now but the zipper may not be as long as it once was.