R1 and L1 not working on 2 controllers

I have two controllers, both which I haven't used for several months. On both controllers, L1 and R1 aren't working, and register no input on the PlayStation. The controller which I've been using still works fine. I've seen several threads about people who've had this issue after controllers sitting, but noone has any solution. Does anyone know what could possibly be wrong, and how I could go about fixing it?

Thanks!