We’ve found the most common reason this happens is because the phone may not be seated properly. Please, give both sides of the dock a gentle squeeze. This should provide a better connection and ensure that the phone is more securely mounted in the dock.
This is something that you may have to do from time to time.
It’s also possible that the phone may take up to 10 seconds to recognize the dock and begin to charge. If it doesn’t recognize the dock immediately, please allow up to 10 seconds for the device to be detected.