Usb Charger Problem

I put a USB charger in my Top Box for charging my phone. At one time it used to work fine but now it seems to have stopped working. When I say stopped it's not dead as it will increase the charge on my phone by a couple of % if connected for a hour or so. So the USB port is still powered, just not enough to fully charge the phone.

I can charge the phone by connecting it to a second USB charger I bought to fit in the front auxiliary power socket.

But why has my Top Box charger stopped working.

One addition bit of info. The power source I use for the Top Box USB also powers a camera which it originally didn't. Could this be stopping the USB Charger from working.

Any thoughts on what could be wrong, how I can test the power output, or indeed fix the problem.