Acrobat Pro DC for Windows (21.001.20138).

Acrobat will not OCR entire large documents (i.e., 500-1000) pages. It will OCR a couple dozen pages then stop, as if the OCR process is complete. No error messages or anything. However, the vast majority of the pages in the document remain unsearchable / not OCR-ed.

BUT, if I scroll to one of the pages that was not OCR-ed, and choose to OCR just that page (as opposed to the entire document), OCR works for that page. This has only become an issue with the most recent version(s) of Acrobat Pro.