We get the same question from time to time from people that bought these for a range of PDAs.
Not unusual to have problems with USB Charge & Sync. cables on suome USB ports.
From what I have been told (Could be complete rubbish)..
In theory, all usb ports are supposed to meet some power output requirements. However, in some cases some of the ports are piggy backed off other ones. A little like having a multiport hub I guess.
Problem is, unless there is enough power in them you run into trouble.
In the case of a hub, an external power supply should do the job.
In case actual ports on the PC, there are usually some ports that have enough kick in them and some that don't (On the same PC that is).