Your FP1U’s USB port (charging port) probably doesn’t sit firm on the phone’s motherboard anymore. You can either try to find a good repair shop who can solder it back in position for you or charge the battery outside the FP1U in an external universal charger device. Repair shops that have done this repair in the past can be found here:
If you want to read more on this specific issue (a known FP1 problem), here’s where it got discussed in detail complete with the specification of a fitting replacement USB port:
Last but not least you should try to clear your USB port from dust that maybe crammed at its ground by now to make sure it’s not just this that disturbs charging.