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. 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.
Apple (and many third-party) Power Adapters rated above 12W support USB PD (Power Delivery). These power adapters can deliver more power by using higher voltages (remember the basic power calculation - Volts (V) multiplied by current (A) equals Power (W)). USB PD power sources typically use 9V, 12V, 15V or 20W - supplying up to 100W - but will default to 5V operation if the connected device does not support USB PD, or the protocol handshake fails. As described, your iPad may be unable to use a USB PD power source, perhaps due to a fault, but continues to accept charge from a basic 5V USB Power Adapter.
Standard USB charging is limited to the standard USB 5V bus. Given your description, your iPad many be able to charge from a 5V USB power source, but may be unable to charge from USB PD sources.
Be aware that many computer USB ports 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 from Apple Support - and request a remote diagnostic test. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively and perhaps easier, 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/gb/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
