LOL... I didn't even read past your first sentence before I went and pulled that link for you... Looks like you've already found my thread.

Are you pulling the first knot as hard as you can and then laying the d-loop material straight alongside the string and tying your second knot without "pre-forming" a loop? That should take care of it right there... In fact, I have to watch it or I'll get my d-loops too short!

Also, your loop material could be stretching... I and several others have found that the camoflauge string has less stretch to it than some of the others.