look at settings / device / battery, it shows how long the charging takes.. Should be around 160 minutes when the battery is empty.. try to take out the charger/connect it again, and look at the messages, in case it shows that the power is to low for charging, it can be a bad usb connector (that is the case with my n9 at the moment) playing a bit with the usb cable can help in that case... (or send the phone out for repair)