How are you measuring your quarter inch seam? You don't measure the seam, you measure the resulting patch. Here's how to check:
http://www.quiltingboard.com/tutoria...ce-t89997.html
also, make sure you are not pressing a tiny fold into the seam. When making these blocks I usually over size the triangles, then cut down. This is also called an Economy Block. If you google this you will find more tutorials and maybe one of them will 'click.