I think 6 days past due is probably too far. There have been cases on here and other sites of up to 4 days over, but if the rest have hatched pretty well together and on time I would have said there was a great possibility that there has been a problem and the chick has died after pipping.
With...
Just had my first hatch in my home made incubator hatch over the weekend and very happy with the 8 little fluffy bundles that are running around the "brooder"(4 foot fish tank). I had to help the last one out after 24 hours as the membrane had go hard and stuck him in place making it impossible...