I received my B5's today (ordered July 25) via UPS.  Both are great, hashing right speeds, etc.  Except I noticed one ran hotter than the other...

The fan in the back (the one that pushes air out) never turns on.  I opened the case up - it's plugged in fine.  I tried swapping the fans to make sure one wasn't just broken - fan in the back still doesn't go.  So it isn't the fan that is broken.  Anybody have any similar issues / any ideas?

EDIT: I put the case back on and decided to let it run for a while, and the fan turned on. Lucky me. Good suggestions though. If it happens again, I'll just connect the fan directly to PSU.