We upgraded from Win7 - Win10 we have numerous users that cannot merge with PDFcreator. When you select merge is onlt saves the last document and will not merge. I tried different versions and the only one I found that worked was 1.7.3
please note that in the current version you need to (after pressing merge on the print job window) you need to either use the "merge all" button or manually select which documents to merge before clicking proceed in the merge window, otherwise it will act exactly as you described.
Did you do this, but it still didn't merge? I can't reporduce the error here currently.