Carefully blow or suck out the connector on the iPad. It is possible something small has lodged itself on or near the pins, thus blocking the connection.
Hm, you're probably not doing this, but just in case. The iPhone charger will only trickle charge an iPad (while sleeping), and will generally say Not Charging while doing so. Same for most laptops. The iPad charger can be used to charge both devices handily. It has a much higher power rating.