Sewing HST's from squares along the diagonal I mark the diagonal and stitching lines with "Quick Quarter" (from quiltersrule.com if LQS doesn't have it). Instructions say to add 7/8" to the finished size of your squares for this method. If you are having trouble, try adding 1" so you have some to trim.
Being way off may be a pressing problem, too. Be careful not to stretch as you press!