There really isn't any kind of tricks with those knobs, there very staight foward in application and since you can turn the right knob, I strongly suspect the left knob was more than likely broken by a prior owner of the car. Anyway, to remove the shelf, I believe you'll need to remove some top clips on each side as well as a screw and 10 mm bolt on each side. Once removed you can inspect it to see if it can be repaired ( I believe all of the parts are plastic so if cracked you might think about replacing it) or replace it with a used shelf from ebay.

The only P.I.A. about this shelf is working the clips back in when reinstalling it.

- - - Updated - - -

See you fixed it while I was typing. Glad it all worked out.