First, make sure the hub is inserted all the way into the USB-C port. If you are using a case, connect it with the included extension cable. If it is still not detected, restart your iPad:

1. Press and quickly release the volume button nearest the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

If the restart does not help, submit a support request.