Thanks for your response. The new fireplace has not been used for any extended period.. so perhaps the flue did not had sufficient time to 'warm-up.Is it possible that the flue itself is in a cold position? If so, it can take a lot longer for the flue to warm up. If the flue is relatively cool, the cool air in it tends to press down on the warmer air from the fire (because the cooler air is heavier) and prevents all the smoke going up, thus forcing it back out of the fireplace opening.
Where I am most domestic construction is masonry, and it is well-known that fireplaces/flues on external walls often don't draw well because they remain damp and cool.
Also remember that if it is new, the masonry will still contain a lot of water. This will take time to dry out and, in the meantime, will tend to keep the chimney cool.Thanks for your response. The new fireplace has not been used for any extended period.. so perhaps the flue did not had sufficient time to 'warm-up.