Happens that you need make up air to maintain draft. If this is conventional fireplace, you need to find some way to allow air (preferably from outside) into the fire to maintain combustion and draft. When the doors are open you are drawing your indoor air up the flue. This works to pull draft, but it is not very efficient. I would guess that no provision is made for air to move through a vent at the bottom of the door or into combustion zone through a vent to outside. In a modular home you may also lack chimney height to maintain draft.
Does smoke clear once fire has been going strong for a while and you shut the doors?