If the shut off valve under your sink isn’t working, it could be because it’s either stuck, partially closed or leaking. If it’s stuck, you can try to fix it with a little bit of WD-40. In most instances, this is a short-term fix. It is important to turn the valves on and off every so often along with spraying them with WD-40. If this fix doesn’t work, then it’s time to bring in a professional.