Quick reply...
It's a complicated matter. Does N900 charge itself from PC, through USB cable? If yes, then does N900 fail to draw current from the bank when N900 is connected to the bank by the same USB cable? If yes, then can you compare {power given by the bank to N900 before the bank switches off} with {power given by PC to N900 while charging}? Maybe, N900 doesn't recognise the bank as charger? Because N900 has to differentiate between low current of PC-charging, and higher current of adapter-charging, and the bank apparently resembles neither of them.
Best wishes. Bumping the thread...
_________________
Per aspera ad astra...