how can I re-enable panel shadow for both top and bottom? CCSM doesn't seems to work for me. Any ideas?
Make sure compiz is activated, then launch ccsm and go to Window Decoration (make sure it is activated too) and on Shadow windows type
any (if you only want shadows on panel but not on windows, type
type=Dock). If it is still not working, reset all the value in Window Decoration, just make sure Shadow windows =
any. If it is still not working, go to Window Rules and make sure it is unactivated.
And if it still not working, make sure compiz is really activated and in ccsm go to Preferences and click on Reset to defaults, that will reset all the parameters of Compiz so if you have a lot of customs parameters you will lost all of them and will have to make them again but it will almost certainly solve the problem.
If you are using ccsm, try adding
Class=gnome-panel to the windows option for decorations. Usually enabling rgba does not affect shadows.
I'm also going to guess you are using emerald? If you are, install emerald-theme-manager, and modify the theme so that shadows are visible again.
If you provide more information I will update my answer to suit.