I had a buddy show up from the neighboring ridge last night.
He brought with him a block of rosin, the kind you'd use on a violin bow.  He said it had really improved his box call.  I tried some on one of mine.  I don't know if it will work any better.  Right now, my season is largely unhindered by the appearance of turkeys, but I keep trying.
Have any of y'all ever tried rosin?