hmmm... It still sounds to me as if there's a flaw in the way you're tying your second knot, and not keeping the string "taut" between the two knots.

Realizing that the square knot you're tying is simply two overhand knots, can you verify that the d-loop material lying alonside your center serving is taut as you're tying your first overhand knot, and after? If you have to, you may have to "pinch" your first overhand knot so as to keep that d-loop material taut until you finish off that second knot with that second overhand knot.

The reason I say this is that your material shouldn't really stretch until you actually start shooting it...

Does that make sense?