First, double check your seam. Remember, you don't actually measure the seam, you measure the resultant patch. Good instructions here:
http://www.quiltingboard.com/tutoria...ce-t89997.html
Next, when you make your HST for the pinwheels, make sure that when you trim them down/ square them up, the diagonal seam goes exactly through the corner. All your HSTs should be pressed the same way, for instance toward the print. When you place the 2 HSTs together, the seams should nestle together.
Here's a you tube video on one suggested method
http://www.youtube.com/watch?v=O18wxk9BZk4