how do I prevent my text fields flattening? (using Zotero)

Hi. I'm using Zotero as a referencing tool for my thesis. I enter a citation which automatically generates an entry in my bibliography at the end of the document.

I'm having trouble (which Zotero say is entirely happening within Word, not their software) where some of my citations are 'flattening', i.e. just turning into plain text, and unlinking themselves from my bibliography. This is proving a real nuisance.

There is no pattern to this: I've recently seen flattened and unflattened fields within the same sentence. I don't do anything fancy with my documents: they're just Word documents saved on a Onedrive file, no changes to settings, no migration into/from other programmes.

Please can you help me prevent any more of my fields flattening

Thank you