Your KYC can be rejected for multiple reasons, including:
1. Blurred images
2. Face unclear in a selfie or Govt ID
3. The face in the selfie didn't match with photo in the Govt ID
4. Not using original documents, etc.
You will be able to find the exact reason when you restart the KYC process from the My Ads > Preview Ad page (please refer answer to the question "How can I start my KYC?")