Just wondering - what size triangles are you using?
They need to be cut from a 3 7/8" square to come out correctly.
Or you can make triangles from 4 inch squares, and trim them down after sewing. A bit of a bother, but then you get perfect sized squares.
Also - did you preshrink your fabrics? I've soaked mine in hot water and then washed gently in the machine on warm.