Exporting Midi tracks problem. (specifically AD2)

Hi guys, When I finish tracking on a song, I like to export my AD2 tracks to a folder I call stems, Then I can import the bounced tracks back into the project and re-save it as 'song name (drums bounced)' That way I can go back and change the original drums if I want. The reason I export the AD2 tracks is to save cpu power when mixing. I tried to do it today and I export the tracks, but when I import them back into the project the clips are blank and there is no audio.
A few Things to mention... 1. The original songs was recorded in sonar x3, 2. I'm working with platinum 'Gloucester' 3. When exporting I have 2 saved templates, one is mono stems where I just have fast bounce selected and they are mono files at 24 bit with no dither and the second is stereo stems which is the same except I have stereo file selected. and 4. It also happens when I select the midi track plus the tracks I want to bounce and try to bounce to tracks.
Anybody got any Ideas why this is happening, It's a major issue for me as I like having as much CPU power available when mixing.
Thanks in advance.
Andrew

bapu
The only time that has happened for me is when the drum audio tracks were muted during the export.

I defo had them unmuted, been doing this for years and never had a problem.