What should I do If my iPad Air only charges one way?

As USB-C cables are fully reversible (and should work either way up) - if multiple cables exhibit the same issue, this would suggest that:

• the USB-C socket itself has damaged contacts, or
• the host device has a fault

Either way, you would be best advised to take your iPad and cables along to the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to hands-on test and assess the items for faults. If an issue is identified, you'll be in the best possible place to explore your available options for repair:

Genius Bar Reservation and Apple Support Options - Apple

Before taking your iPad in for repair, you may be able to save yourself some time and hassle by cleaning the charge port.

Use something soft, like a toothbrush or some wooden/plastic toothpicks. Many times debris can get packed into the charge port and prevent charging.

See this article for more information: If your iPad won't charge - Apple Support