Do you leave your iPad connected to its Power Adapter for extended periods? If so, this is not of itself an issue - as the iPad will optimally manage battery charging. When connected to a Power Adapter for an extended period of time, such as when used in Kiosk or PoS (Point if Sale) applications, iPad will change its charge management strategy - and reduce the maximum battery charge level to about 80% of maximum capacity.
This strategy is intended to extend the life of the battery by reducing chemical ageing that occurs when the battery is continuously maintained 100% charge. As soon as a “normal” charge/discharge pattern is resumed, charge to full capacity is restored. If your iPad has paused charging at ~80%, simply disconnecting the Power Adapter from the iPad for several seconds may trigger completion of the charge.
Temperature is also a contributory factor in charging being paused; the iPad is designed to reduce the charging rate if the temperature is too high for optimal charging. Summer high temperatures may cause this protective measure to be triggered. Try moving yout iPad to a cooler location whilst charging.
More information about iPad’s battery management can be found here:
About iPad and iPhone Charge Management Feature – Apple Support
These support pages should help with troubleshooting more general 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. Be aware that 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.
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 Get Support link at top-right of this page. 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/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