It's not that it isn't charging when it's on its that the power output is equal to or greater than what is being supplied by your charging unit. Well that is if it shows the charging symbol, which I assume it is. Anyway when it's off nothing is using power so everything it gets is working to charge the battery. I'd get a new cable or charging unit. Some of the best ones I've gotten were at dollar xyz, dollar general, dollar tree, family dollar, etc. Avoid Duracell brand. In my experience they are pricier, work well for awhile but don't last and break easily, IMO. Your cable or you plugin unit my be damaged and not working efficiently. Supplying low voltage to your device. While shopping for a charger if possible look for something that's at least 1mA. It's a nice mid range charger. I think you can find them up to 2, which will charge really fast but could also damage devices if they don't have built in regulators, most do these days but if you're not sure... Just saying. You'll want to avoid the .5 and .750s you'll only be disappointed in the length of time needed to fully charge.