Smoothing between auto lip sync

Hi all!

I have used Auto Lip Sync to create my animation for the mouth. However, I can't figure out how to create a smooth transition between each lip frame. I can do this when drawing basic shapes/frames with the create shape tween. However I can't seem to do this after lip sync, the option is greyed out. 

Any help on how to create  smoother trasition between frames is much appriciated!

Thanks

Ben