black soot deposits in sitting room with wood burning stove

Hi.
I have a solid fuel wood burning stove in my sittingroom. I regularly have to wipe the tops of metal curtain rails in this room due to the build up of sooty deposits. When the stove is in use there is no apparent smokeyness in the room nor smokey smell yet the presence of the sooty deposits must indicate that smoke is escaping from somwhere. In the past we have had the odd problem with downdraught causing release of smoke into the room. That hasn't happened yet this year yet I again noticed the build up of sooty deposits again after recent cleaning. I've checked the fire cement where the flue meets the stove and where the flue meets the chiminey. Both are fine. I'm concerned that the stove is releasing smoke into the room at a slow steady level indiscernible to the room occupants and therefore poses a health risk. There is a single vent in the room. The chimney is not straight (two bends). It was last cleaned about a year and a half ago. I was considering a rotorvent to try to improve the draw. anything else I'm missing?

One final thing is that the sooty deposits seems to be of a higher concentration on the metal curtain rails over a bay window on one side of the room. For example we have a bookcase on the other side and there is little or no soot desposits on the top of that (strange).

thanks

tom