Most of the time this is due to a damaged cable or dirty connections. You could try a different plug, like the one you use in the car, but it's my guess that your connector on the end of your cable is loose and not making good contact. This could also be due to dirt and dust build up in the port or connector. If you take the device in to your local corp store they will often clean your port for free and check it if asked. Cables are cheap and they all do about the same thing or with little difference. I've found good cables at Big Lots, Dollar General, Dollar Tree, and Family Dollar. Some of them didn't last long but I blame that on my kids being rough on things and the quality, 65/35 respectfully. If you can just try a different cable and see if you still get the warning, if so see about getting your port cleaned.