I just had a look inside the jobby, after prising it open. The wire from the circuit to the micro USB has only two cores, ie it only has the regulated 5 volts and ground connections. Maybe the phone "expects" something clever in the other two connections of the micro USB to tell it what to do.
My sister had a similar issue with Motorola Razr phone which would not charge when connected to her computer, apparently there was some sort of software fix for this. I don't know if it was a driver for the PC end of the link or a code download into the phone. I'll see if she can shine any light on the issue.
Rans6.........