I had the problem myself, so i'll post a link, which shows the fix.
It worked fine by me.

https://www.youtube.com/watch?v=yW_pt56F_fM&feature=youtu.be

Another "fix" for Avernus stopping before his objective is to either: Have Shale use Rock Toss, a Mage use Fireball, or Warrior use shield slam. This will nudge him back to the summoning circle, and He'll start.