I had this exact same problem last night on a project. I actually discontinued the bounce several times and restarted the process again thinking that something had hung up.

 In my case the project had two soft synth bounces and some pretty heavy rendering going on with mastering plug-ins and such. I also had numerous dual sends on several tracks.Lots of busses.Not sure what was the prime contributor to it but I found that if I just waited it seemed to process first and then mix down. It took some time and what I thought was the program starting over was actually multiple individual writes.

 After waiting the track exported perfectly. This was the first time I had such an intense mix, so maybe the intensity of the mix had something to do with it. I was also bouncing through my master buss loaded with Ozone 5. This is probably not recommended. It would have been better to make a master and then import to master with Ozone. That's a lot of number crunching going on at once.

 I would suggest waiting until it completes.

All great advice.  I froze all the tracks individually.  The issue seems to have been that some tracks have multiple sections of audio in them and these were the tracks that had to be processed over and over.  If I had let it run when I wanted to export the entire mix, it probably would have eventually got there.  Strange that I've never come across this before though.

Cheers for your help guys