Are you using the Apple wall charger or just a computer USB port? I'm sure you're aware that a regular USB port cannot supply enough current to properly charge the iPad - well, at least, it charges very slowly. The Apple wall charger and USB cable have a much higher current rating.
It's quite easy for the connector at the bottom of the iPad to become ever-so-slightly mis-aligned. Several Forum members have reported having this 'problem', which can often go un-noticed. It's probably worth checking that the connection to the iPad is properly 'seated' in the iPad's socket.
Let us know how you get on.
Tim
Scotland