saving files

I am not crazy after all. I was beginning to thing I was losing my mind. I opened a file yesterday, made a few edits and saved it. When I re-opened later in the day, my edits were not there. So I did my edits again thinking I must not have saved the file. Reopened the file again later in the day and ... no edits. I thought I was losing it.
Today ... same thing. The edit I was doing was changing the stacking order of the layers. I had a highlight box below a bitmap image on several slides. I changed the order so the highlight was above the bmp. I saved, closed and immediately re-opened the file to see the file did not have my changes. Only when I merged the bmp onto the background, did my edit get saved. There is only one copy of the file, so I am not editing one file and opening another.