I've only just recently had this problem, at first the shadows started flickering, I thought that gmod fucked up on something and that if I restarted the game it would be fine. After the restart, no shadows at all on props, every other game the props work perfectly, but the shadows on props will not work at all on Gmod, no matter what shadow setting I put it at, and no matter what map that has props...
Here are some screenshots
Notice the barrels
(http://i1245.photobucket.com/albums/gg600/Fuzsion/2013-01-27_00002_zpsad7c5214.jpg)
Some more screens
(http://i1245.photobucket.com/albums/gg600/Fuzsion/2013-01-27_00003_zpseea44aad.jpg)
Table has no shadow at all
(http://i1245.photobucket.com/albums/gg600/Fuzsion/2013-01-27_00004_zps077c485c.jpg)
Along with the chairs
(http://i1245.photobucket.com/albums/gg600/Fuzsion/2013-01-27_00005_zps558545b2.jpg)
Help would seriously be appreciated, this is annoying the shit out of me.