I've realized that on some of my jeans the zippers often slowly slide down, even when I'm just walking from one class to the next. It is so embarrassing and I often find myself trying to re-do my zipper without anyone noticing.

The weird thing though is that it's only the zippers on some jeans (2 of them are skinny jeans- does that have something to do with it?)

Has this ever happened to any of you guys? Do any of you know what causes the zippers of some jeans to fall down and the zippers of other jeans to stay up? I need advice on what I should do to help keep the zipper up for good. 
I would really appreciate any and all advice!