Smoking Kohlangaz Karisma GR 3243515 model?

Have had this coal effect gas fire serviced and the engineer advised me that there is a problem in the chimney causing the fire to billow out smoke into the room. Have had the chimney swept twice and was advised that there is no blockage or problem with it.The fire is still smoking.
Has anyone a clue. For example can the coal nuggets be incorrectly placed. Would appreciate help as we have recently sold the house as seen and obviously the incoming owner will be expecting the fire to be in working order and safe.