You could have a faulty charger. See if you have another one to use and see if it works better
Make sure you are using an iPad charger and not iPhone charger or USB. Neither of those will give the iPad enough power to charge efficiently
Swap out the cable if you have another and see if that helps
If none of that helps then your battery may be going bad. You can make an appointment at a local apple store or authorized service provider and have them check it out. If the battery is under warranty the replacement may be free, if not then it runs around $100 for a new battery