It is very rare that there is a problem with the stove, it is always the chimney or wet wood, yours is not drawing properly. The chimney sounds high enough so it will be:
. Air is entering the chimney elsewhere which is providing an easy route, rather than being pulled through the stove. Check for leaks, particularly if there is an ash pit below, or if the chimney is being shared with another opening.
. If the chimney is not lined you are going from a 6" flue pipe into a much larger void and losing suction. The answer to all your problems will be to drop a flue liner down the chimney and connect it to the flue pipe and the top of the chimney.
. The 90 degree angles are really not helping. If you have to have them, take the flue pipe vertically off the top of the stove for as far as possible (keeping a safe distance from combustibles) before going through into the chimney void.
. Don't burn wood which is >20% moisture, preferably <15%.
Hope this helps.