It is a puzzle, particularly if you were previously shooting the A&H to the same spot as the Dakota, which I assume is the case. 8" at 15 yards is too much to adjust by changing the strike material. I can normally adjust 3-4" at 20 yards that way. I often shoot a longbow to the left of where I shoot a recurve, since they are often not cut past center like my recurves are.
While there are many reasons I might miss to the left, the only reason I miss to the right is when I pluck the string, which is pretty obvious to me when it happens, and doesn't happen consistently. You seem to have eliminated arrow spine as being the culprit. Have you tried holding the longbow vertical to see if that moves the point of impact to the left? Not that you would want to have to shoot that way, but it would be another piece of information.
If it were a recurve, I would suspect limb twist, but that is rare in a longbow. I'm not familiar with the A&H, but I've heard it's one of the best longbows out there. Any possibility the upper and lower limbs could have gotten reversed?
I really enjoy my Dakota too, although I mainly use mine for 3D.
Sorry I couldn't be of more help, but I'll be interested to see what the problem turns out to be.