Sequential Max - No sound from track 4

I have recently got a Sequential Circuits Max which does not produce sound when the keyboard voices from track 4 (though on certain settings, it may play an aspect of the voice, such as white noise or a modulation freq).
This can occur when pressing any note on the keyboard, as the Max alternates what track it voices from in an almost random fashion.
I have cleaned the inside, nothing appears to be obviously broken.
Any help is appreciated.