this is a common issue with the Nexus 7 1st gen tablet from what I can remember - try contacting Asus and seeing if they will have a look at it.
you could also try a different charger encase the one you have is damaged. Finally if you have the 2nd gen Nexus 7 this supports wireless charging so see if you can find a wireless charger and try charge it that way.
Nexus 7 1st gen is 2012
Nexus 7 2nd gen is 2013