Apparently, hardwood products can decompose fairly quickly in a potting mix. That wouldn't be a big problem, if the bacteria breaking them down didn't require nitrogen to do so. From what I understand, if the hardwood mulch isn't already pretty far broken down, then you'll likely wind up with a deficiency.
That's what I've heard digging around other forums. Supposedly, that's why many nursery-oriented potting mixes contain pine bark fines, and not hardwood bark fines. But then, on the other side of the coin, there are these guys, who basically grow their plants in rotting wood.
http://permaculturenews.org/2012/01/04/hugelkultur-composting-whole-trees-with-ease/
This makes me think that adding some kind of slow-release nitrogen fertilizer might make your potting mix work.