Bottom Knot Keeps Slipping Out

I have a Freeman 370 that I recently made about 1200 bales with.  This was my first crop with the baler and overall I am very happy.  However, about 1 out of 20 or so bales that come out of the chamber hit the ground and the bottom knot comes apart and the string falls off.  (I say bottom because the baler gives the bales a 1/4 turn so its actually the left most knotter when standing behind the baler looking forward)  All the knots look good until the bale expands when it comes out of the chamber

 

As I read the different forums I see that this problem seems to be related to a loose billhook or improper twine disc tension.  Can someone explain to me how to tighten a billhook?  Also can anyone talk about how you determine proper twine disc tension?

 

My knots really look good and I feel like overall the machine is functioning very well but I would like to see that bottom knot stop coming apart when the bale hits the ground or when the harrowbed picks up the bale to stack it.  

 

Thanks in advance.  

Andrew

Bottom Knot Keeps Slipping Out