Try to slow down when sewing. Are you using rotary cutter and rulers or just cutting
with scissors?
I don't use a 1/4 inch foot for sewing HST's. I use my open toe foot so I can see
where I'm going. You draw one line corner to corner and then sew 1/4" away from
that line. It's easier to draw that 1/4" on each side then you know where to sew.
Also, I prefer to cut bigger squares, draw the lines, sew, press then cut the
triangles. Press open then trim to size. Hope this helps.