Zippers fail due to dust and grit in the zipper coil which, in turn, cause the sliders to spread apart. If the zipper coil sometimes opens up behind the slider, it may be possible to crimp the metal "jaws" back closer together and restore function. Slowly and carefully crimp/pinch the flared part of the sliders (top to bottom, both left and right sides) closer together. Pinch, try, pinch, try until the teeth start to lock.
Clean your zippers with mild soap and water to remove the grit which caused the problem.
If that does not work, it will be necessary to replace the sliders. We are happy to send you new ones (no charge).
Here are the repair steps: