IDP showing "Extraction tools not available for this document"

Hi!
I am trying to employ the Intelligent Document Processing application to extract data from identity documents using Appian as the vendor.
In some cases, on the reconciliation interface appears an exlamation triangle showing the following message:

By examining the process model, it should be due to the "Start Doc Extraction Smart Service" and "Reconcile Doc Extraction Smart Service" nodes, however their behaviour is not explicitly clear.
Therefore, my questions are:

1. Why does it show the above message? What does this behaviour depend on? Only on the OCR and on the quality of the images or are there more influencing factors?
2. Is there any detailed source of information that explains in detail how the two aforementioned Smart Services and the Appian Document Extraction algorithm work?
   (Yes, I have already read the relative documentation and I am looking for in-depth information)

Thank you in advance for all the help that you will be able to provide.

Massimiliano