Some PDFs downloading without comments

morning all.

We're using document cloud to mark up PDFs while we all work from home in Aus (we produce weekly retail catalogues). We upload a few PDFs per week without issue,  But sometimes PDFs which have comments on in the browser download blank (the PDF content is there but no comments or mark ups when we can see them online).

We're trying to work out why,

Does the document size affect this (the current one that isn't working is 93mb while my other draft which worked was 16mb?? Surely this shouldn't matter!?

This seems to be the only difference in the PDF.

We're using Acrobat reader DC on PCs, with chrome as a browser for Document cloud sharing/marking up.

Thanks