well... the only thing i can think is that maybe after sewing a seam, when you turn it out to sew the next piece on, maybe it isn't a tight turn. that doesn't make sense does it?? my poor brain isn't giving me the words to explain.
let me try again.
when i am paper piecing and sew a seam on the paper side, i always flip it over and press it right then. gives me a nice flat seam onto which i sew the next piece of fabric. maybe you are leaving a little looseness in that seam? that is the only thing i can think of if you are following the lines precisely. that looseness might give you a distorted block once the paper is removed. or could you be stretching the material during the removal of the paper? be sure to shorten your stitches so the paper is perforated closely. will make it easier to remove the paper.
(unless your printer is distorting the copies somehow?)