Have you purchased an Apple USB-C to 3.5mm Headphone Jack Adapter - or something else:
https://store.apple.com/xc/product/MU7E2ZM/A
Some third-party adapters may not be designed for iPad - and this may be the source of your problem.
If you do have an Apple Adapter, there are two fundamental possibilities. Either your Adapter may be faulty, or your headphones are not correctly wired for Apple devices. In general, there are three main wiring standards for 3.5mm TTRS connections - and they are not interchangeable:
- CTIA (Apple)
- OMTP (Nokia)
- Camcorder AV

If you are unable to resolve the problem, you would be well advised to 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, Adapter and headphones:
Genius Bar Reservation and Apple Support Options - Apple