I think I've found my problem. I designed this in EQ5 and when I do a template for the sashing strip it tells me its 10x1 in. So I added in the 0.5 in seam allowance that I would need for each strip, therefore I cut my strips at 10.5x1.5
But when the blocks are sewn into rows, the "middle" blocks on the row only measure 9.5 inches wide, not 10 inches. So I'm guessing maybe I should have cut the strips at 10 inches? But then what do I do about the "end" squares on the row where the strips wouldn't end up wide enough to cover the whole square?