NH 479 haybine roller help

The roll drive chain is on the sprockets but no longer engages when machine is on. The belt runs the reel drive and whatever but the sprocket next to the biggest belt sprocket just sits there. Not sure if this is clear at all but it's been awhile since I've had to work on this machine and I'm not sure if there's a broken key or something and manual sheds no light on the problem.

NH 479 haybine roller help