It appears there is only one spring on the T/S switch for the cancelling mechanism, so if the R works okay, it would make sense that the L should also work.
Have you made any moves on the steering column moving it in or out lately? Since the C cancelling clip is attached to the steering wheel, and the steering shaft is fixed, changing the column location could change the orientation of the clip to the switch. This is just a thought.
Also, if you rebuilt the top end of the column, the top bearing has a separate insert for the inner race to keep things tight. Does your steering wheel have any side to side play at all?
As John already stated, the cancelling mechanism is purely mechanical, so if these items all check out, it may be time for a new switch.
Let us know what you find.