I have cleaned the contacts there is no visible dirt, and the keyboard still does not work.

Are you using the Magic Keyboard for iPad, or the Smart Keyboard? While both of these keyboards connect with the iPad via the Smart Connector contacts, only the Magic Keyboard for iPad has pass-through charging from its spine USB-C port.

Begin here:

Use your Smart Keyboard Folio or Smart Keyboard with your iPad - Apple Support

Assuming that you have followed all the advice contained therein, if the issues persists, your next step is to force-restart your iPad - as doing so will often resolve an isolated software or device connectivity issue:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

If you continue to experience difficulties, you will need to seek assistance directly from from Apple Support. 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 and keyboard:

Genius Bar Reservation and Apple Support Options - Apple

MustafaAlmosawi wrote:

... I either get the ‘this accessory is not supported’ message (but not even that often) or nothing at all. It used to be intermittent that I would get the message, close the folio and put it back in the typing position and it would be fine, but now it’s persistent.

See >>> If you see an ‘Accessory may not be supported’ alert - Apple Support

If you are using a Smart (folio) keyboard, the alert is typical of a faulty folio keyboard. The electrical connections that pass though the flexible fabric hinges typically fracture - this usually resulting in errors and/or the on-screen notification that you see. 

The Smart folio keyboard itself is not repairable. The only remedy is replacement of the keyboard. As you appear to be within your one-year limited warranty, you need to contact Apple Support or your retailer.

You will find the Get Support link at top-right of this page. 

Alternatively, you can initiate a support case using the excellent Apple Support App. If not already installed on your device, the App can be downloaded from the App Store:

https://apps.apple.com/gb/app/apple-support/id1130498044

A further alternative is to visit the Genius Bar at your local Apple Store, where the technicians will be delighted to assist. When visiting the Genius Bar, a reservation is recommended:

Genius Bar Reservation and Apple Support Options - Apple

If you connect the keyboard to the adapter and place the iPad Pro on it, does the iPad charge?