Action opening all the files at once

I have been using an ancient Photoshop7 to batch process 17000 PDF files per month.  That was great until something changed in PDF recently that now crashes the old photoshop.  So I installed Photoshop2020 and made the same action.  It opens the PDF, flattens it, adjusts levels, turns to paletted image and saves as gif in another folder and then closes it.  But when I run the action on 2020 it starts opening all the files at the same time and doesn't seem to do any of the other steps.  Opening 17K files at once is not going to work.  What has changed?  How can I make my simple action behave like I expect, processing 1 file at a time?

Thanks