edit:Yiwu DingQiang Import and Export Co,.Ltd Update:August 17 2019.
The zipper is often the least durable component in any garment or type of equipment. Most often the zipper fails to close due to a worn or bent slider not being able to apply the necessary force to the sides of the teeth to cause them to interlock. This problem can sometimes be redressed by using small pliers to carefully squeeze the back part of the slider together a fraction of a millimeter. This can compensate for the wear of the slider. The slider is typically made as a magnesium diecastwhich breaks easily. It is necessary to reduce the force on the pliers before it can be felt that the slider actually gives in. If it is not yet possible to successfully close the zipper, the pressure applied to the slider should only gradually be increased. Another way to reduce the gap of the open end of the slider is by preparing a small block of wood by sawing a slot into one end so that it fits over the upper arm of the slider. Then a hammer can be used to exact a force onto the slider by carefully hitting the wood.
When the protective coating of the diecast slider has been worn off by prolonged usage, the material can corrode. The corrosion products are usually metal salts which can accumulate and block the slider from moving. When this happens the salt can often be dissolved by submerging the slider in vinegar or another mild acid. Otherwise the slider needs to be removed and replaced.