The USB ports on some PC's/laptops will not be giving out the same power as on other machines which then might not be enough to charge if the reader battery gets really low. You could even find that some ports on the same machine would give out different current depending on how it's all wired up internally. As said above though plugging into a powered USB hub should be getting you a decent enough power for charging, but you'll still always get a quicker charge from the stand alone mains powered charger.
I did see something in the brief manual for my PRS300 that said something along the lines of it taking an hour before the screen would come on to show that you were connected via USB if the charge was to get really low on the reader.
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