Eileenkny - not sure which triangles you're referring to, but I did notice that sometimes it is the SIDES of the triangles that are positioned to cover the area. It can seem a bit confusing to set them this way, but a dry run by folding a quarter inch of fabric and placing it in position before stitching helps to troubleshoot whether the fit will be right or not.
The larger white triangles fit on the outside ends of units 17, 16, 15, 2, and 6.
I was careful to fold back the paper to trim the last sewn seam before trying to place the next triangle. Most critical of all is pinning the intersections of the star points when assembling the final sets. That one gave me the most trouble.