Dollars to Donuts says it's probably that DT line. Try it out with a WF and I bet you will get the distance back. After you perhaps prove it to yourself though...put that DT back on and lawn cast it until you are hitting those same marks.

It also may be that you need to let the line unroll a bit more on the back cast. I find that I can get away with more of my terrible casts when I'm fishing with a beefy shooting head; the more "limp" lines have a lot less tolerance for my sloppiness.