Im no expert but which operating system the Mac is on ? which midi interface are you using (do you have the latest drivers for it ?) Is your sibelius pgrm fully compatible with your mac os version or you interface ? Check also your midi cables...
And finally( but that would be for experts) maybe there is a setting on your disklavier (model ? )-that is not properly selected...
The fact that another keyboard has a problem also seems to me to eliminate the disklavier proper cause...
Lets see what else comes up?
-alain