The locking bolt is spring loaded toward the locking plate. So corrosion, crud, etc could prevent the spring from pushing the bolt rearward (toward the locking plate) far enough to lock the steering.
As reference: The ignition switch has 5 positions from full CW to full CCW they are: START, RUN, OFF, OFF-LOCK, & ACCESSORY.
The die cast rack is designed to mechanically pull the bolt back from the locking plate when your are in any ignition key position except OFF-LOCK and ACCESSORY. In these two positions the spring should push the bolt into the locking plate. (You can see the small coil spring on the locking bolt in the following picture.
Most literature that I have reviewed says to NOT lubricate the locking bolt where it passes through the turn signal switch housing. This might be so that in cold weather any grease or lubricant could prevent the spring from pushing the lock bolt into the locking plate.
It seems pretty obvious, but have you tried to clean out the locking bolt bore to insure that the bolt is free?
Otherwise, you will need to disassemble the steering column head to inspect the locking bolt parts as shown.
Here is the address for a number of Steering System papers that relate to the 69-76 Camaro/Chevelle:
Scroll down to the 69-76 Standard Steering Column Section. You will find three Disassembly & Repair Papers, two Disassembly & Repair Pics, and a Standard Column Blowup Pic.
These papers and pics should provide you with nearly all the information that you might need to take the column apart and determine the cause of your lock bolt problem.