Check the model to see how thick the wall is and what you have the extrusion width set at. For instance If I have a wall that is 1mm thick and my extrusion width is set at .4 then there is a .2 gap. I set the extrusion width to .39 and it fixes it. Or, change the dimension of the thin wall to be a multiplier of the extrusion width.
You can see it in S3D print preview.