Your should start here - with basic troubleshooting of potential charging issues:
Charge and monitor the iPad battery - Apple Support
If your iPad won't charge – Apple Support
Having established that neither your Power Adapter or charging cables have are obviously defective, you would be well advised to visit the Genius Bar at your local Apple Store - where your iPad, Power Adapter(s) and charging cable(s) can be assessed and tested by the Apple Technicians.
Of this is not possible, you should contact Apple Support for assistance. It is possible to remotely test some aspects of the iPad hardware. Whilst remote testing is not a substitute for “hands-on” assessment of the device, it may identify a fundamental hardware issue.
Apple Support can be directly contacted using the Get Support link (also found at top-right of this and every Apple Support Communities page). Alternatively, you can initiate a support case using the excellent Apple Support App:
https://apps.apple.com/gb/app/apple-support/id1130498044