(Bumped, because I don't feel this justifies a new thread.)
I've had a similar problem in Fluxbox, but was unable to find the cause of it except that it was connected to a setting in the init file I was using at the time so I simply replaced it with another one, restarted fluxbox and the problem went away. Because the entries in the two init files were in a different order, it would have been very hard indeed to track down which setting in the init file was responsible for the problem but if anyone else knows what the problem might have been I'd be interested to know about it (a Google search didn't prove fruitful).