When I log out from gnome , and log in to kde4 everything is ok, but switching back is a problem - after choosing gnome goes to kde again!
It repeats several times , and then logs to gnome.Same on desktop and laptop,also with different WM.
Why is this happening?Its verry annoyng!
I’ve noticed this as well… On my system after I log out of kde and go back to the login screen and change to gnome I’ll get logged back into KDE.
However to stop this from happening, all I have to do is enter my password and THEN change the WM… It then lets me login to the desired system. If I change the WM before entering my password I’ll always get dumped back into whatever I used last.
And no, I have no idea why this happens 
Hope this is of help…
Had the same the other way round, i.e. Gnome reluctant to make way for KDE. Eventually settled down though.
Sometimes work ,sometimes not.Feels like lottery 