Does it charge off a wall transformer?
If it charges elsewhere then I guess its not a fault of the device?
More likely that the powerbank expects negotiation and sansas don't negotiate..
Anecdotally sounds more like an issue with the powerbank because my powerbank charges my clip+ and clipzip.
if you are handy you could probably rig a few resistors to a cable and make a cable that charges your device from those power banks
used to be just as easy as a dedicated charging port places a resistance not exceeding 200 Ω across the D+ and D− terminals
minty boost explains some of it on the handling of iDevices
https://learn.adafruit.com/minty-boost/ichargingmy guess is that your powerbanks are not recognizing the 100ma? initial draw or need a device with negoiation..
My powerbank tells me current draw .10 A at start rises towards .2 after a bit and works fine with my clipzip