A few more tips.
Make sure that when you place the ruler on the fabric to cut that the line is over the fabric. If you only bring the line to the fabric edge, it's going to be short by the width of the line.
Starch and press the fabrics first. Starching after can cause the unit to shrink.
Make sure the seam line is at 1/4 inch. Use a thinner thread for piecing. It can make a difference.
When the points really matter, I will make the 1/4 inch line and stitch on the line. Try to sew from the side that so you can see the intersection. Stitch right on the point or one thread width past on the inside of the seam. When you flip and press, it should be good.
Don't get frustrated---just keep going!
Check your pressing technique. Press not iron. Don't iron with the pointed tip which can distort the unit also.