Not really, but from your description, you hit the rear THIRD of the lungs with your shot. I'd say the first shot KILLED it, but IT didn't realize it before you fired the second shot. It just needed some more time to bleed out, like being hit with an arrow. BTW, GOOD JOB!
I like hearing more guys using their bowhunting skills with a firearm, it makes it more fun and challenging.
It sounds like the bullet expanded, but possibly not to the full potential of a LEAD NOSED HP. The copper all the way to the nose MAY cause it to deform less on a 'soft' target such as a whitetail's ribcage. I'd say it would work WELL on bigger game with heavier ribs with the EXACT same shot.
I would guesstimate that if you had shot point on the SHOULDER, that it would have opened up more and torn the lungs to pieces on the spot. But obviously the bowhunter in you said to AVOID THE SHOULDER. I do the same thing!
I'd be curious to see what a lead exposed HP would have done in the same instance. Chances are it would have been very similar, only possibly a little quicker...It's anyone's guess, but I'd say you didn't have a 'failure.'