On the Google Nexus 6, the center of the wireless receiver is lower than the center of the phone. If you have an AT&T model, the center of the wireless receiver is at "U" of the NEXUS logo on the back. (Other models are probably the same.) If the centers are not aligned, it may not charge or may not maintain constant charging speed. Therefore, you may need to adjust the position of your phone to align the centers.
If this does not fix the problem, the device may be defective. Please contact us for warranty service.