I am trying to program a key FOB using the GM Tech II tool..... and though I have tried several different key fobs for our application, it will not program................. what am I doing wrong?

Which vehicle are you referring to? Do you have the original key fob for the car ?
On the back of my original key fob- Park Ave for instance, there is a GM UTA number...and also a FCC ID Number.....
The problem may be that these numbers have to match for the fob itself?

It'* for my son'* 2005 Pontiac Grand Prix.... we bought it a few months ago and it did not have a remote with it when we bought it..... so we do not know.

In that case it may best to contact the dealer with the VIN # and inquire about a factory replacement fob and requirements.
Alternatively, an auto locksmith may be able to provide you with an aftermarket fob that could be programmed for a more reasonable price.