DrPhil,
You don't say what year car you have. Latches vary with model year. For '65/'66, its easiest to take the latch out, clean it up, lubricate it and re-install.
Before you do however, look inside door shells. I usually put a flourescent drop light inside the door shell so I can see the insides better. Disconnect the lock control rod. Operate the latch manually to see how stiff it is. It should be firm operating but not stiff. Does it now latch & lock? If it does, check the lock cylinder and see that it moves freely with no control rod attached. Examine the angles of movement and see if they are consistent. Note: it makes a world of difference if the lock cylinder lever is moving somewhat fore/aft vice up/down. If there's a lot of fore/aft movement, you wouldn't get full travel of the control rod. Perhaps the lock cylinders are installed in the wrong doors. In '65/'66 models the lock cylinder levers point to the front of the door.
Jeff
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