Do you have logging enabled for fluxbox? If so can you check the log and see if there are any relevant messages? If not, it can be enabled by changing the startup entry in ~/.fluxbox/startup to something like:
exec /usr/bin/fluxbox -log "/home/steve/.fluxbox/log"
Also, can you check /var/log/Xorg.0.log for messages (less likely, but just in case...)