First thing I would do is verify that I have exactly 1/4" seam. or if the pattern is calling for a scant 1/4" it's slightly smaller. You should not really be loosing size unless it's in your seams.... There is a thread on here somewhere about checking your seam size.
Click on search above and enter 1/4" seam or 1/4" scant seam. Read through a few different posts. There are ways to make sure you're keeping good seams. After you do that, make a few more samples and see if they don't end up right at the 6" you're trying to get at. Some patterns really need you to be right on or it's a bear later.
Hope this helps