What method did you use to sew your HSTs? When your HST is done, do you check that it is square and that each half of the triangle goes perfectly into the corner. Your HST on the bottom left seems to be spot on - the one on the bottom right shows more of the solid green than the pattern where it comes to a point.
Then you seem to have taken a bigger bite than a 1/4" when sewing the two sides together.
You may want to try a paper-pieced version for a change. Or, sew them oversized and trim them down. Don't forget, if you sew them oversized and trim, you must align the 45degree line on the bias where the triangles meet. If you just square the block without that alignment, you'll end up right back where you started.
Don't worry, HSTs can take a while to get perfect.