A rare hardware conflict in Windows may be the root cause.
So you've got your keyboard connected and it even shows up in the list of music devices on the Settings screen inside Synthesia. It also shows up in other apps (like MIDI-OX). But no matter what you try (say, swapping the In and Out cables on your MIDI adapter) it never responds when you strike a key. You may even have a Logitech webcam connected to the same computer. Does this sound familiar?
As far as we have been able to gather, there is a location in the Windows registry that tells Windows how ALL multimedia devices should be handled by the system. Now, the default value seems to be correct for MIDI keyboards. Unfortunately, the installers for a few other devices out there (at least certain Logitech webcams) have been known to change this setting. This is what is breaking compatibility with MIDI keyboards.
This is definitely an obscure problem, but changing things back to the default value seems to get keyboards working. We've heard the same answer from a handful of keyboard manufacturers.
So, we created a small utility can detect the problem and fix it. It must be run with administrator privileges.
If you're unable to run that utility (say, because of an overzealous anti-virus app), this registry file also contains the same fix. After downloading, just right-click it and choose "Merge".
You may need to restart your computer after running either fix.