Why are the R1 and R2 buttons not responding?

I bought a few brand new PS3 controllers from Amazon. Two came in a pack and one works fine. The other one works fine except for the R1 and R2 buttons. The buttons aren't broken, and when I took it apart, I tried using the sensors to push the buttons and it didn't work. I did the same for the L triggers and it did work, so I know it's supposed to work. All the connections and ribbons look fine, and it's brand new, so it's clean inside. The sensor pads and everything look fine, too. I don't know what else to do. Any advice?