Are you using a modern "digital" stereo ??
If so, they require a full time battery connection as well as an ignition connection, to keep the clock and tuner set.
If these leads get crossed, then you'll have to reset all those functions every time to start the car.
Or if one of the leads isn't even connected, you may not be getting power to your stereo. ie. Ignition to battery lead and no power to ignition lead.
As for factory radios, that's a completely independent circuit.
One thing I have found with G.M. ignition switches is that over time, the "turned back" position on the key tends to fail. Not sure if it's just a worn out point in the switch, or dirt build up that isolates the point.