can't add subdocuments in master document (word 2013)

[Glxblt76]

Dear fellow Word users,

I'm using Word 2013 in the Windows 10 64 bits environment .

I have a recurring problem with building master documents from several subdocuments.

My problem is simply that if I create the master document, add several subdocuments, close the master document, and then later re-open it and want to add new subdocments, I simply can't ! The following screen capture precisely shows the problem (it's in french, but the button I frame with the red rectangle is the "insert" button used to insert subdocument link in the master document)



The only quick fix I found to this problem was to re-create a new subdocument. But then I had to go through re-numbering of all figures, tables, equations, references, and re-uniformize the formatting, and re-generating the lists of figures and tables.

I don't want to do it again so I ask to you: how to avoid this problem to occur, or how to make me again able to click on "insert"?

Thanks in advance for kind help!