Combining PDFs never finishes

I've seen a couple similar posts and followed many troubleshooting suggestions already, but nothing has solved my issue - I'm trying to combine 4 PDF files within Adobe (also tried using Adobe Acrobat online), and the files will upload and can be previewed, but when combining, the process never finishes. I've re-started and checked the software version is up to date. I've also tested other PDF files, which have no issues combining. So, I reduced the file sizes and made sure there was no protection but the same issue still occurs regardless of how I grab the files or drop the files. There is no error message or failure - the process simply doesn't finish. I even tried adding one file at a time and they all have the same problem (and are from the same source). Oh, and I already checked to make sure the "save as PDF portfolio" option is not checked. Any other suggestions?