Keep in mind that not all USB ports are fully powered. I understand that this problem is especially prevalent with some USB "Hubs"... On my PC for example, some of the USB ports will cause my iPad to warn me that it is not charging, it actually IS charging, but just very slowly. Most of the other ports give a full charge and no warning from the iPad. Usually the wall warts DO provide ample current, but it would be a good idea to try some different USB ports and see what happens.
Initially, my Range Extender took a very long time...5-6 hours...to charge, using a proven Wall Wart. Since then it's been much quicker, but it still takes longer than the main battery, which will recharge from about a 20-25% state to full charge in an hour or so....