Wall Chargers what make them work or not work?

Why do some USB Wall Chargers have the ability to charge the e200 series players while others don’t? What make the ones that work different then the ones that don’t?

I have moVox 2GB Media Player made by Centon and it has a standard Mini USB Cable and a little thing that plugs into the wall that has a USB part in it but it won’t charge my Sansa e250 but will charge the Coby MP620 I got at Biglots, my Garmin Street Pilot c340,my NET10 Samsung t401g, my LG 8360 Cell Phone (But the phone pops up a message that the charger is not recommended but still charges anyway) and my Creative Zen Vision W.

If I plug the Sansa e250 into my PC when it’s running Linux it will charge and it will also charge when plugged into my Laptop running Windows 2000.