from my experience, turkeys are funny about shock gobbling. I bring a peacock call with me, and right at first light, I try my owl. If this gets no response, I hit 2-3 notes from the peacock. Usually this high pitch will get an immediate response from a turkey. Also, turkes don't always shock gobble before light, no matter what you use.
Hope this helps. Also, try a single hen decoy. It was the advice of some vetrans here that put me onto it. It works very well.
You can't kill'em sittin' on the couch!