Can anyone explain this annoying occurrence.

I am making a move with about 30 mins of total video.    I have added perhaps 6 short audio clips (total audio perhaps 1 minute).    I find after some editing work that some audio clips seem to have "moved" from the spot in the movie that I placed them.     So, they were in the correct position, but later on I'll discover they are not where I put them.    It is very tedious to re-position them.

What can be causing this??