Why is my power/lock button not working?

I do alot of screen replacements, so I'm familiar with the iPhone. But this one has me stumped. The power button acts like it has lost it's spring. I ordered a new button and replaced it and still have the same problem. With the actual metal button removed you can push the inside part of the button and it works. Any ideas if I'm just putting the new one in wrong? Or do I need another part?