Thank Lance for your answer! 

I did set up my Kerf and margin between parts (of nest) at the same value as you recommend but still it doesn't work so far. Additionally, when I read through Autodesk guided for Camduct, they specified that the "margin between parts" value has to be at least "equal" the "Kerf" value (so if my Kerf value is "1.5mm" then I should set my "margin between parts" value to be at least "1.5mm", obviously it can be "2.0mm").As the photo below: 

I reckon, the point is the margin can't be less than the Kerf because we won't get the right dimension of the parts since the "tool width" gonna be over the gap between parts. 

So we have to at least set the margin to be greater/equal to the Kerf so that after doing nesting with "common line cut" property being choosen, the nest will automatically remove margin value down to equal "Kerf" value (if we set margin value greater than Kerf value before nesting) and keep the gap between parts (if we set margin value equal the Kerf value before nesting). 

But, down to the original problem, still I tried your recommendation and it hasn't worked yet. Any idea Lance! 

Thank you for your reply .