If the mains charger & usb charger connections use a common lead, try a different lead (there may be a beak in the wires). Otherwise it is likely that there is a fault within the phone, preventing charging of the battery.
Mrs Hymie’s phone suffered this fault – with my electrical know-how, I have rigged up a charger that makes direct electrical contact with the battery pack terminals. On the plus side she has a spare battery to keep one charged but it is a pain to lash-up the required connections to the battery pack.