file keeps escaping my fence

One of my MS Excel files keeps "escaping" the fence I put it in, and it ends up on the desktop.  I move it back into the fence and double-click to open it, which takes a while (some lag time).  After I finish my work and save it again, it ends up on the desktop and not in the fence.  Does anybody know how to fix this problem?  I don't seem to have this problem with other Excel files, or anything else.  Thanks!  Mary Mack