I too had a charging problem. After all night charging (I thought) it was about dead at 3% left. Saw a post about Non-Certified cables may not charge the Nexus 7s. So I un-wrapped my never used, SHORT stock cable and noticed the tabs on one side of the micro end seem to protrude more than my other micro cables, probably holding in better BUT the cable may be slightly different in other ways...I plugged it in and my Nexus is charging faster than it did before.
That's all I know, using the Stock Nexus 7 USB cable SOLVED my charging issues. I read about other issues as well, possible defective chargers, charging chips etc. I hope the cable issue is the main culprit here because it is easy to fix. I DO think other micro USB cables will charge, they must, I have used 4 different cables since I had my N7 for over two weeks now, but the stock ASUS USB cable shipped with the N7 is charging my N7 much faster than what I have seen before.
Cheers!