Front fender rubbing on tire
The problem is that the metal fender (or fender bracket) has some spring in it and wants to spring back placing constant tension on the plastic retaining bits that hold the slider. Ideally, the sliders should not be under any tension or compression - they just should keep the fender from some side to side and a little of fore and aft movement.

If you have a metal fender: I would bend the fender so that it's closer to the wheel even when under no tension:


Take off the sliders
Push the rear of the fender against the wheel with enough pressure that you feel the metal bending
Release and repeat until the fender springs back to about the position you want it to stay in.
If it keeps springing back even though you've pushed it against the wheel, you might need to take the wheel off to get some additional space to push
Once the fender is about where you want it, then (put the wheel back on and) the sliders back on


Plastic fenders: If the fender is made of plastic or wood, then there is a metal  L-bracket underneath the brake calipers bolt that holds the fender on. This bracket needs to be bent just a tad so that the fender is at a better angle viz a viz the wheel. It's easier to bend this with the fender removed as you can crack a plastic or wood  fender. 

Finishing Touches: After you've done all that, you can put some polyurethane or hot glue on the sliders to help them grip a bit more. I like to use glues that I can easily take off again. One brand I like is ShoeGoo. It is incredibly tough and tacky but you can also get it off hard plastics and metals if you need to readjust the fenders again.