Typically it's a problem with the source.
If you're plugging the USB end into a PC or older Mac, iPhone charger, typical cell phone USB AC adapter, this will result. But....fear not! Even THEN it IS charging, but at a much slower rate. This has been acknowledged by Apple and most folks are now aware of it. If you've no other option and probably won't use the iPad at night, go ahead and plug it into one of those things and it WILL charge. By the morning it should be close to, of not fully, charged (make sure the computer doesn't go into "sleep" mode or shut down on its own after a while of non-use, as this will cut all the power).
For proper rate charging, you will need a USB AC adapter that is capable of 10 amps output. MacAlly makes a great slim dual-USB port wall AC adapter that's small and folds up for easy carrying which supports iPad charging simultaneously with another USB item. And its cheap. That's just one example. Sosche is another good brand. There are many out there. Ditto for DC (car) adapters. Just make sure the specs for the one you're considering specifically states it will charge an iPad.
If for some reason your iPad won't charge at a proper rate from a 10 amp USB adapter, try a different iPad charge/sync cable as well. They're cheap and easy to find, especially on line or at Fry's.
If you're near an Apple Store or any other store that sells iPads, for that matter, take yours in there and plug it into a known good cable and adapter from one of the display units or something. That's a good test.
If all else fails, there is the slim chance that your batteries in the iPad or the charging circuit is suspect. In that case, the folks at Apple WILL help you.
Michael "Spam, spam, bacon, eggs and spam. Hold the bacon and eggs." Sent from my iPad 2 using iPF