I have had this happen with my Nintendo Switch Pro controllers before as well. The first step is to take the back plate of the controller off, open it up, and carefully lift the triggers and rubber pads up. If you see any debris, or corrosion, use some isopropyl alcohol on a q-tip to gently wipe it off of the contact. If this still doesn't work, you may need to get a new controller, unless you're willing to replace the internal board.