I had logged into a KDE4 session with compiz working fine, then hit ‘Switch User’ to get an XFCE session, which I did, then I tried to run ‘compiz --replace’ from within the XFCE session (it didn’t work – I ran it and nothing happened) then I tried to run ‘xfwm4’ after doing ‘killall compiz’, didn’t work either.
I ended the session and went back to my old KDE4 session, of course the compiz had been killed by my ‘killall compiz’ by accident. So I tried to run ‘compiz --replace’. It killed kwin and then made the desktop flash once and then nothing – no titlebars and I wasn’t able to move windows around.
I have tried with a different user, still nothing, and I’ve tried uninstalling and installing it.
I haven’t tested this much since I don’t switch sessions often, but from very quick testing, it looks like you may have to log out of the session using compiz if you want to get full compiz-ness in another session. If reinstalling hasn’t done anything, try going back into the desktop effect app and re-enable it.
Also, when you’re not able to move windows around in compiz, it means the window move compiz plugin has been disabled. If you don’t have titlebars, then you may want to run emerald --replace&. It may not be a bad idea to restart the computer in this situation.