It sounds like the locking collar is loose. There's a roll pin and a slip on cover that goes over the snout of the collar when it's installed on the shaft properly.
There's also a piece inside the column bowl where the key tumbler is that is spring-loaded. It catches on one of the slots to lock it. That could be worn out or loose as well.
Another possible cause is misalignment - collapsed steering shaft, bearing insulator is shot, coupler is shot causing movement, breakaway pucks on the mounting collar, etc. All this stuff can wear out over time and cause binding or excessive movement which will lead to stuff like this.
Only other thing I can think of is that someone took it apart before you got it and failed to put it back together correctly.
It's not hard to take it out and take it apart. It does not take any special tools or skills.
Pop it out and see what's going on.
Here's a pic of the locking collar. You can see the depression in the slip on cover where the roll pin is.