Many things could be the cause...
1. If you're not programmatically creating your own
ChannelSets, did you
provide a -services command line argument pointing to the
/WEB-INF/flex/services-config.xml
file when compiling your Flex application?
2. Are you sure that the hostname and port used to load the
SWF are the same
as the channel endpoint URLs? If they are not the same, do
you have a crossdomain.xml
policy file setup in the webroot?
3. Did you check the server logs to see whether there were
any configuration
failures reported on server startup?