Not all blocks can be squared up. You need to try and figure out where you are adding or loosing dimensions. try and measure each subsection of the block as you go. Make sure that you really, really know where that darned 'scant' 1/4 inch is. Don't rely on your foot or plate markings.
Here's a tutorial on how to check your seam:
http://www.quiltingboard.com/t-89997-1.htm
Also, you need to make sure that your cutting is accurate. Use the SAME ruler for the entire quilt and if that's not possible, use the same brand of ruler.
And if all else fails, get the book "Setting Solutions". She gives lots of ideas on setting blocks of different sizes!