I have freed up probably 2 dozen drop rate controls on BX series over the years, I have never had to disassemble one.
As D2Cat stated, spray it with penetrating oil and let it soak overnight. I usually remove the knob and put a small crescent wrench on the shaft to wok it back and forth.
They stick because dirt and moisture build-up where the shaft goes into the housing. There is an o-ring a few millimeters below the surface resulting in a void exposed to the elements.
Easy to take apart, hard to access. and I agree (to a point) that spraying with a penetrant is the easiest solution but not a permanent one. Some models (not the BX) have a jointed shaft, not a straight shaft and the joint also becomes seized. It is a bad design in general and the only solution to the issue is regular lubrication and regular 'adjustment' of the lowering control valve. I 'adjust' mine every time I use the tractors. Give it a turn open and closed and return it to it's original position.
Issues arise because of the location and that location collects 'junk', water from washing and other debris and eventually locks the shaft or the needle valve and especially on cab tractors, access is very limited.
The used M9 I purchased last spring was locked tight but because I stripped and refinished all the tin, I was able to completely access the all the underlying components including the valve, linkage and transmission case and I 'cured' the issue by lubricating the valve and actuation shafting.
Non issue now.
