Why are the Candidate AI ratings not generating all the time

Summary:

We have enabled the AI Candidate Scoring feature, as well as Dynamic Skills. The ratings are appearing from some candidates (if the data is available), but I cannot get a Skills rating to generate, even though there are Skills on the Requisition and candidate profiles.

This screenshot shows that 2 of the candidates have Skills on their profile, but the Skill rating has not generated, as it says there is no data. But there is anther candidate who does have Skills and a Skills rating.

What is the Skill rating based on? Do the candidates have to have parsed their CV? If so, how would this work for internal applicants?