Are you talking about Side Trigger Button#28? I have sent one (TPMC-3X) back for this already. I have 10 at one project and each feels different when pressed. Im hoping i can determine which ones will fail by how they feel out of the box. One is spongy and doesn't really make a clicking sound and the other clicks easily.

I think its like when you press a VHS tape into the VCR from the side. Eventually it warps it just like pressing the side trigger from the bottom corner instead of straight on. Thats a theory i am working on anyway :).