The problem probably occurred when you sewed your seams. You should be sewing a scant quarter inch seam, and if the two pieces are lined up correctly, the points will not fall at the edges of the fabric, but a quarter inch in from the edge. That way when the next quarter inch seam is taken, the point will fall right at the edge of the seam and not get sewn/lost into the seam allowance. If yours did not come out correctly, it is also possible that you did not line up the two pieces correctly. Often with triangles and angled pieces, a quarter inch point of one fabric has to hang over the edge of the other fabric when you line them up. Sometimes there needs to be a quarter inch point of fabric hanging over at both ends of the seamed edge---on one end it's the top fabric hanging over, and on the other end it's the bottom fabric hanging over. The problem you had was most likely lining up the fabrics incorrectly or not sewing an accurate scant quarter inch seam. There are lots of internet videos you can watch to see how to line up various triangle pieces to each other.