There can be a number of reasons why documents can be rejected for verification. The most serious ones being tampered with documents and fraudulent documents.
The most common issues with document rejection are these:
Bad quality image (blurred, out of focus, not lit well)
Expired documents
No photo of face on ID.
Digitally modified photos
Cropped images.
Personal information isn't correct.
If there is an issue, the verification service, SumSub, they will send a request for new images or other documents.
You can also contact support to help you with this.