Lee,
I’m just wondering if it’s possible you have overlooked something when setting up for the audio mix down.
Are the locators set to include the parts you want?
Did you check the correct track in the left side of the Audio mixdown dialog?
Do other tracks render properly?
You are using an instrument track? Or rack instrument? The process is different for each.
The flat line problem is often fixed by using the little slider in top right corner of the project screen.
Hope it helps