These support pages should help with troubleshooting battery and charging issues:
Charge and monitor the iPad battery - Apple Support
If your iPad won't charge – Apple Support
Troubleshooting generally requires substitution of the Power Adapter and cable for another known-good item. Most models of iPad require a Power Adapter rated at 12W or greater to reliably charge.
If you are using a third-party cable or Power Adapter, substituting these for the items supplied with your iPad is strongly recommended - as these will conform to the required specifications. Your iPad Pro will likely have been supplied with a 20W USB-C Power Adapter and corresponding USB-C to USB-C cable. Older models of iPad Pro may have been supplied with a 12W USB Type-A Power Adapter, with a Lightning to USB Type-A cable.
5W Power Adapters intended for older models of iPhone are inadequate to charge an iPad; if the iPad charges at all, charging will be very slow. Be aware that many computer USB ports also cannot source sufficient power to charge an iPad. Unless explicitly designed for charging connected devices, computer USB ports are typically limited to 0.5-1A @5V (i.e., 2.5-5.0W).
If you continue to experience difficulties when using an appropriate Power Adapter of 12W or greater, you might be best advised to seek assistance directly from Apple Support - and request a remote diagnostic test. You can contact the Support Team using this link:
Official Apple Support
Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044
You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad, Power Adapter and cable(s):
Genius Bar Reservation and Apple Support Options - Apple