You are apparently storing your modifications in the file normal.dot. Something is interfering with your ability to save changes to this file, probably a poorly-written Add-In. Another possibility is that you are in a corporate environment where someone in IT has frozen this file.

First option, the one I use, is to put the modifications in other files. Everything except style changes can be stored in a different Global Template.

A second option is to directly open this normal.dot file as a document on the screen, make the changes and save the file. Normal Template in Microsoft Word - How to Open

The third option is to figure out which Add-In is causing the problem and disable or delete it. How to find out whether any Word Add-ins have been installed The Add-Ins that have recently been spotted to do this deal with Blue-tooth functions but Add-Ins written by Adobe and even Microsoft have caused this problem in the past.