It's looking to me like the triangle isn't the right size.
What does your pattern say?
I would start by making a triangle at least 1/4" all the way around, sewing it to the desired pieces and then checking if they measure correctly.
Also, have you emailed the pattern designer?
I know patterns can become misprinted...so maybe they have a correction some place.
Also, use some scraps or your ugliest fabric to test this since you have limited amount for the quilt.
Kirsten