I've had the same issue with my 6600 when I'm NOT using the accufeed system (when I do, no problem). I've also had it hesitate to feed (or need help) in the middle of a seam where there are bulky seams to rise over (again, only when not using accufeed).
Overall I love my 6600, but sometimes it's like driving a Lamborghini in the rain.
On the other hand I also have a Huskvarna (purchased when they were still made in Sweden), and it can feed pretty much anything I throw at it. Like driving a Volvo.