Originally Posted by JanTx
Before you sew them together square them - find the smallest one and cut the others to match that size. It could be cutting, it could be the fabric is stretching, or it could be the seam allowance isn't consistent. Cut them square and constant, then sew them together.
Just be sure you don't cut the points off any triangles - mistake I made. :) Depending on your pattern you could also add sashing to make the blocks all the same size.