If the chimney has been cleaned properley, then it is safe to assume that there is no blockage.
Now you say you live in a tenement on the top floors (I guess you mean flats??). How old are these flats, as chimney construction has varied over the years.
Ie: is it a traditional stone chimney, flexible flue liner, or clay liners???
If it is a traditional stone chimney, then it could possibly be one of two things:
1. Downdraught - If you live next door to a large building or strucure, wind can be forced downward and cause your chimney to puff the products of combustion back into the room. (solution, fit an anti downdraught cowl on your chimney pot)
2. Sweeping the chimney has dislodged a mid feather in the main chimney stack, blocking your chimney (the mid feathers seperate the main chimney stack into several, so each fireplace has a 'seperate flue path'
So you need to confirm what type of flue you have.