OK, I needed to use sidechain again yesterday, once again it would not work in the project I had open. So, I copied the vocal track I was working on into a new porject. Set up the signal path Vocals>Patch 1> Aux Track. On the patch channel I set up an effect chain in the Pro Channel, delay>eq>reverb>PC Bus Compressor. I turned on the sidechain button on the Bus Comp then sent the vocal to this from the send channel. Voila, it worked perfectly, the Comp got rid of the vocal and only allowed the delayed vocal to come through, just as I wanted.
Then I saved this as a track template, opened up in the original project and it did not work, the sidechain was not receiving anything. I then went back to the project where it was working, and..... it did not work anymore. No matter what I did I could not get it working again, I even tried setting up a new chain but no luck.