Latent or low service bars after you remove the mid-frame from outer frame is due to contacts within the outer frame not making the proper contact with the board. You can test service outside the frame which is one to two bars and very difficult to make calls. Once refit to the frame properly the service is boosted. If you are having service issues, its more than likely the frame. Take apart and check the inner frame and reassemble. Check service before you replace the back screws and glass battery door.