First, double check your block and sash measurements to make sure something isn't off. You might not notice a 1/8" difference by eye but by the time you get to the end of the row you're gonna be MAD!
Second, don't forget to "nest" your seams, even though there is sashing between the blocks. Usually you'd nest where the blocks meet each other, but in this case you're going to nest the whole seam, except for the width of the sashing. (I find this to be more accurate anyway.)
Finally, pin like it's going out of style! Don't rely on a pin at each corner; make sure you are pinning every couple of inches to give yourself a chance to make a correction or ease in a little extra fullness.