As a follow up to my last reply: I decided to create a new project with the (SERENITY - Serenity - Slow 16th Pop Ballad) that I had added to the previous song where the guitar part was reported to be missing. This time RB had no trouble in creating the guitar midi part!

So ... going back to my previous song, could it be that the original style was all RealTracks? Because that is the only significant difference. Surely the loading of a new style will allow the generation of all the parts of that new style, even if the the prior style had all Real Tracks.