Power & Sensor Replacement Problem

I have a Iphone 4 At&t phone that had a broken screen and power button would not work. I replaced the screen and everything worked fine and once my power & sensor cable came in I replace it following the tutorial and video on here.

At first I thought that I had killed the mother board since the phone would not boot up. I connected the mother board on another iphone and it worked fine. So i decided to connected just the charging dock, and screen cables to the motherboard can connect it to my laptop. It turned on so then i decided to connect the front facing camera. That also worked, now I connected the power ribbon to the board and it doesn't turn on, but when i do disconnect it i turns on!.

So what can be causing this? Am i missing something?