madmanmike:
This sounds a bit familiar as I had the same problem with some props in the past.
I balance my props and have encountered the same thing you describe.
Some prop's center hole is off center and will be only be off by a fraction.
I balance them, then remove the prop from the balancer and flip it and remount it in the balancer.
You'd be surprised how many props will not be balanced once flipped.
By flipping I mean having the back of the prop facing you when you balance it initially, then have the front of the prop facing you to double check the balance.
Also if the reamed hole isn't perpindicular to the prop it could cause probelms as well.
I hope this helps.

Tom