I started new Prussian campaing and utterly crushed Poland-Lithuania by taking 3/5 of their provinces and destroyed Saxony and most of their armies, they won't accept peace unless I return every single province...
On the other hand I was glad to see Austria attacking me because I expanded east (like the adviser says when starting Austrian campaing, that you might have to attack Poland-Lithuania to prevent Prussia and Russia from growing)
For some reason no one asks or accepts peace anymore.