Why doesn't my spare key unlock the car door? Main set of keys are locked inside on 2006 Chevrolet Equinox

My main set of keys with the unlock/lock key fob are locked inside the car. I have a spare key (just the key - no lock/unlock button) which fits in the lock but doesn't turn to unlock the door. Why not? Any other options to unlocking other than calling a locksmith?

Asked by for the 2006 Chevrolet Equinox
Only reason it wont work is, IT'S THE WRONG KEY!! Think about it. I am sure you tried it in all locks right?
It's the same key that starts the car - so it's not the wrong key. It fits in the key hole, just won't turn. There is only one key hole - on the driver side door - none of the other doors have key holes, including the back. Locksmith said they could make the key work in the door. Weirdest thing.