Something you might check is for mud on one of the rollers. My BIL just got a 568 this yr and put net on it. He was talking about it the other day that once in a while it is picking up the net on a roller from mud build up. He has only made a little over a 1000 bales so far this year so he hasn't really got a handle on all the quirks of this new unit. He and my FIL have had 530's,535's,566's,567's and now this 568 and he has had about everything you can imagine happen to them but this is the first one he has had net on.
You could also be on to something with the knife that cuts it as in my opinion it should cut it off sqaure and nice so it would be even for the next bale. Hope this helps.. 8)