[Sascha]

you possibly have a filterscript running, which has a "return" at OnPlayerCommandText... this causes that the server stops checking for more commands, when it searched through the filterscript ones..
so unload all filterscripts that are running and then try it again... load the fss each by each and try the gamemode commands again... when they don't work anymore you have the fs that is causing the problem (remove the "return X" at OnPlayerCommandText of that filterscript