As there are many different models and ways to attach rests, there are inevitably lots of reasons why it's not staying in place! Here are a few: • The feet are too far apart. Try narrowing them and see if it's still possible to slide on the rest. • The shoulder rest is too extended. Try turning the feet, so they're closer to the body of the rest. • Some part of the mechanism is loose. Make sure all screws are tight. • The spot where you're attaching the rest isn't compatible with your posture, so you're pushing the rest too hard when playing. Experiment with different angles and set-ups. • The plastic on the feet is breaking, so the rest doesn't have enough grip.