It could be any number of things, not enough KE, bows not properly tuned, etc. Spitfires are slower to open than some other mechanicals, and they may have contibuted to the poor penetration, that and the fact that they are larger in cutting diameter than your muzzys are probably the biggest reasons why. Spittfires are very good heads, but you need a well tuned bow and at minimum 55 ft/lbs of energy.

TAKE YOUR KIDS HUNTING AND YOU WON'T BE HUNTING FOR YOUR KIDS