You may have to walk (top down) the entries in the Content panel while the Properties dialog is opened on the Tags tab. For each "Figure" entry is there Alt text?
Similarly, walk the Tags panel.
Thanks- I've posted again, sorry for the repetition....this happens frequently, not just one document, and the most frustrating part is that USUALLY you can click on the link to a missing TH or figure without Alt text and it takes you to the offender....in these cases, you click and get nothing. In a 200+ page document it's not practical to walk through.. ANy suggestions?