Some additional thoughts to ponder:
My own opinion is that Duraflame or Northland type logs really are not very good to burn. I don't know what temperature they burn at as compared to dry hard wood but, it doesn't appear to be as hot and you do need a temperature high enough to create the updraft in the chimney.
If your house is well sealed, then you might be encountering a problem of not having sufficient air for the fireplace to work properly. A very simple experiment will tell you if this is the case. Crack a window or door open and light the fire in the fireplace. All the smoke and odours should go straight up the chimney once there is sufficient heat in the chimney to produce the updraft.
We had a very large fireplace for the size of the house and this worked wonders.
If the problem is that air is being forced down the chimney past the closed damper when there isn't a fire in the fireplace, check that the damper is clean (even if a chimney sweep claimed to have cleaned it), and make sure that it is closing properly. If those two items check out, you MAY benefit from a chimney cap that looks like a tube on its side. I never needed one but am told that it sets up a negative pressure in the chimney when the wind blows thereby drawing air up the chimney instead of blowing down the chimney.
Hope this helps