From time to time doing stairs you can get a wick effect it has happened to me a couple of times, as i understand it you can have dust collect under the nose of the carpet on the stairs and sometimes on the tread, when cleaning it can appear to clean well, but i have seen it myself, as it dries the soiling wicks to the surface the two times it happened to me was on a very worn carpet, i had vacuumed thoroughly first but made no difference, its a disaster waiting to happen and not something you can avoid.
Of course it could be something totally different.