It might be trickle charging via your PC. That would mean it's charging at a lower rate than the iPad charger's usual 10w, but it's charging. Some PC and laptop USB ports offer 5w charging. That prompts the iPad to indicate that it's not charging.
You can easily test by using the original charger and plugging into a wall outlet. If your iPad starts charging, then your PC USB port is charging at lower than 10w.