Midi Tracks missing in Audio Mix down 6.5

Hi Everyone, I ran into a slight issue that just popped up a couple days ago. I was rendering a song for export in audio mix down to mp3 and I played back the file the midi tracks were missing, only the instrument and audio tracks were I even tried to convert midi tracks to audio by arming a audio track to capture the audio to no avail.
A little help would be appreciated to get through this dilemma. Thanks!