power/sleep button not responding after cable change.

Hi, i have an iPhone 4 which won't turn on unless plugged in to charge. The actual button was making the click noise but no response. I changed the proxy/power cable for a brand new one and the proxy sensor works but the power/wake/sleep button still fails to work. im guessing it could be something to so with the actual metal button and hinge? is there something specifc to look for. I was wondering if there should be a spacer or similar in there that i may of missed. I'm confident in stripping and reassembling an iphone 3/3gs/4/4s but there are only so many times i want to take it apart.

Many thanks in advance