There is a secret that no one has told you about triangles. The points do not match. You must be VERY careful to keep a 1/4 inch seam allowance. Measure the seam allowance on your pieces to be joined , you will see where the intersection really is. Triangles do not match up like squares and rectangles. The easy thing to do is to trim off the point of the triangles. There are triangles trimmers galore. See fons and porter for example. Otherwise. measure your quarter inch on adjacent sides of the triangle and cut off the point. Someone else may be able to do a graphic or find a site to show this. The secret is " the seam is a quarter inch in from the point-the point is "extra". I had the same frustration with triangles but eventually it "clicked".
Rita