Has your iPad been repaired/manipulated from the inside before? Dropped? Water damaged? Have you been using a cheap charger? Answer those to help figure out the issue.
Otherwise, here's a troubleshooting guide for the most common issues:
1- The charging dock could be A/dirty or corroded, B/unseated due to a drop, C/Damaged:
1-A/ Gently clean the charging dock (where the charging cable connects) with a tooth-brush dipped in 90%+ isopropyl alcohol while iPad is off.
1-B/ Be warned that you could easily break the glass for this one: use this guide to open the iPad. Look at the battery. Is it swollen? If not, unseat the connector, clean it with IPO and a toothbrush if you like, then re-seat it and test. If battery is swollen, it could be that the battery is acting out. You will need to replace it.
1-C/ Try a new charging dock.
2- The data and/or power management circuit on the logic board could be acting out due to a number of causes such as those mentioned at the beginning. I had a similar case where that circuit was damaged and the iPad would charge from the laptop but not the wall charger.