You can spray the prong with some wd-40 to see if that works, if not pop that black plastic trim piece off that's around the handle,most like going to have a bent rod that's hooked to the handle.
possibly dry & rusty. lubricate well with spray lubricant like silicone (not penetrating oil - it evaporates) other chance box has rusted or stretched like V. try a shim on latches - like washers on bolts between latch & box
