Links to source document aren't updating

I'm a little perplexed and frustrated with some issues I'm having getting a document to update links. I have:

1 source document -- protected -- in our M drive (not accessible to all employees)
15 linked documents -- protected -- in our S drive (accessible to all employees)

The source document contains sales numbers for the staff which then calculates their bonus and is locked so that employees can't see each others bonuses but allows the manager to only have to update 1 document instead of 15.
The linked documents (1 per staff member) contains a summary of just their bonuses and each is password protected using a combo of their name and SIN numbers so no one else can access it.

All documents are in .xlsx format.

What is happening is when the staff attempt to open their document to see their bonuses they are just seeing an error. I can't seem to recreate the error tho. When I open their documents they seem to open fine, fully updated. I've checked the linked documents and each one is set to update automatically in the security settings.

Could the fact that the source document and the linked documents are saved on different servers be affecting the updates?
Could the password to the source document (they don't have this) be affecting the updates?

I'm at a loss.

Thanks in advance for your help.

Thank you FDibbins --- I'll be testing this again next payroll update which should be any day now. I'll do the test you suggested and message back. Thanks!