This could be a couple of things:
1) USB cable not plugged in all the way.
2) You are using a 3rd party cable NOT supplied by mophie.
3) Your juice pack is not receiving as much current as it needs to charge your iPhone. Make sure the wall adapter or charger you are using has an output of at least 1 amp, like the one that came with your phone. Also, try charging your iPhone and juice pack separately overnight for 1-2 nights and it should fix the error.