My recording so far has been limited to patches from a USB sound module but I decided to check out the patches on MC6 Synth Rack, so I inserted a Sound Center track, selected a patch, and recorded a passage. On playback, 80% of the notes do not sound. They appear on the piano roll but they don’t play. Why?

I don't have MC, and I'm not aware of what is a "Sound Center Track". I assume you inserted a soft synth. or is your
Ketron in the picture for creating the sounds?
 
There are (at least) two possibilities. My first suspect is "MIDI buffers - use xxx ms buffers". in the Options/Preferences (I don't know which name MC uses for the settings drop down). Double that figure.
Too small a MIDI buffer is the most common reason for randomly skipped MIDI notes.
 
Secondly: If you have written notation that goes outside the range of the instrument in question, you'll get only
silence for those notes.

I'm assuming he means "Cakewalk Soundcenter" which is a softsynth.
 
did you load an instrument into the Soundcenter?  you have to load something by double clicking on an instrument, there's no default loaded.
 
are you using MIDI tracks you recorded in MC or are you using MIDI tracks you downloaded from the internet?  if downloaded, there are probably PATCH data in the MIDI data of the track which you'll need to delete
 
did you insert an INSTRUMENT track or MIDI and SOURCE tracks?  make sure you have the output CHANNEL, BANK and PATCH information set correctly for the Soundcenter.  if you used an INSTRUMENT track that info needs to be set on the properties of that track, if you used MIDI/SOURCE tracks, then set it on the MIDI track.