I'm shooting at widow kb x 51@28" it's RH as well. Ok I shoot the mfx 500 with the 75 gr insert and 125 tip and they bare shaft great. Now I grabbed a 400 spine of the same brand and insert and tip and it flys nock low, it looks like its fly at like a 45 degree, sound crazy? But the left and right are fine, just nock low, I've tried two nock points to and it still does it??? Could someone please help me out! Thanks

Juan,
  Try putting this in the shooters forum, you might get better results.
Denny

I would try putting a bunch more weight up front. If you are using the same length, weight up front on a 400 spine as a 500 spine arrow, more weight up front on the 400 spine will make good things happen.