Trouble with inbox list on Windows Vista?

My inbox is generally alphabetized in either ascending or descending order, but every once in a while it gets all jumbled up where nothing is in alphabetical order, which makes it difficult to find specific messages. Sometimes it corrects itself, but when I try to do it, nothing happens. I usually go to view, sort and then to the drop-down menu which gives the option of subject, and sometimes that works and sometimes it doesn't. Why is this so erratic, and what can I do to stabilize it so it stays the way I want it? I have Windows Vista home edition and Windows Mail.