If you only got 6" of penetration on the two you killed and the mature one that you lost, I'd say it's time to revisit the tackle.
That should be plenty of bow for this sized game, that leaves the arrow. A few things I'd be asking myself:
- Have you confirmed arrow tune with a bare shaft or paper? You can loose a lot of power to bad tune.
- How many total grains is the arrow? More is more.
- Being really honest, how sharp is the broadhead? It took me a few years to really understand how sharp these things could get.
I'd focus completely on any weaknesses in my gear before I'd worry about how tough my intended prey is.
There is my two cents, aside from agreeing with a ton of good stuff up above.
Thom