Issue - When zippers have been worn down, the sides of the zipper can become unaligned and offset. If the zipper is coil, the two sides can sometimes be realigned.

Technique - Bring the slider as close as possible to the unaligned area. Then move the slider away, gently pulling on the side of the zipper that's bubbled, trying to skip teeth. Bring the slider back down and repeat until the skipped area is fixed.

This technique is a permanent repair. However, if there's any hesitation that this technique will cause damage, remove and replace the slider.