Why does Word not save my document?

This is really frustrating and I need to fix this problem. I am writing a four hundred page novel and sometimes I sit at the computer and edit for hours. When I leave, sometimes I come back a day later to find that all my editing never saved. I press the "save" icon consistently throughout the editing process and at the end AND press the "Save As" just to see it save a second time. Sometimes it saves, sometimes it doesn't. The days that the document doesn't save come at random. So what am I doing wrong?