Can't convert scanned medical records into OCR files?

I have a 2,562 page hospital record that I need to make searchable. Usually these large files all convert to OCR ("Recognizable Text") like magic with Acrobat Pro DC with the enhanced scan tool. But this one only converted partially - the scanned material, like dictated operative notes or H&Ps remained as images, though the other material from nursing flowsheets, for instance, did convert to OCR, and headers and footers converted to OCR (like the hospital name at the top of the pages).

If I print out one of the records in question, like an op report, then re-scan it on my own scanner and save it as a digital PDF file, I can convert it entirely to OCR. But I don't want to have to do that with the entire file.

What's going on?

Thanks for any help.

WBD, MD