Due to a limitation within the Add-on Platform, copying and pasting between Word documents is not currently available. We know it's a giant pain - especially for those working in large document. We are very actively looking for a way around this but at this moment - this is a current limitation of SmartCite in Word. The action of copying and pasting deletes the link between the reference in your library and the inline citation itself when pasted into a new document. This is what we call a NULL CITATION. You may notice a box around the citation after pasting, but it is not linked to anything anymore.
To see whether this is an issue in your document, click on the OPTIONS menu in the right SmartCite panel. You'll see a secondary menu appear that shows the option to SHOW NULL CITATIONS. If you click on this, any inline citations that are affected will be highlighted in red.
Right-click on the null citations and select the option to "Remove Content Control". If will leave the red highlight, but flatten it to plain text. You'll then be able to insert new citations.
If you are already in a situation where you have cited sections of your papers in multiple documents, we recommend updating the style of the document you planned to copy from and change it to APA so you can see the author name/year to make it easier to find and replace after copying it into the primary document. As you replace the flattered citations, those will take on the style you have set in the primary document.
It's worth noting that you can copy and paste within the same document without issue. You will need to go to Options > Refresh references so they can be properly linked in the document.