It's a pretty pattern and looks correct to me. Can you post pictures of your fabric pieces and the tools you're using to cut the diamonds? There's two easy ways that I can see for this pattern to go wrong. I don't see any seam allowances (there's room for mistakes to happen here when cutting odd angles) and maintaining a perfect or near perfect 72* angle on the diamonds would be tricky at best without some sort of jig. It wouldn't take too many small errors to throw the whole thing off.
Rodney