First, make sure your strip set is carefully pressed so it's width is exactly the same across the whole strip and all the strip sets are the same width. Next, make sure that you are cutting the triangles accurately. Are you using a triangle ruler/template or the 45 degree line on your ruler? When you go to sew the 2 triangles together, do they match pretty closely? If not, your cutting may be off or the strips are different widths. Finally, when you sew the triangles together make sure your seam is consistent. Sometimes you veer off at the ends, or you are distorting the bias edge.