If the coarse knob drives properly it is most likely not the tension screw on the focuser bottom. In this case it is far more likely that the preload on the gear drive is not sufficiently tight.
This page may not be exactly the same as your focuser, but pretty much all of generic Crayford 2 speeds (including the Moonlight) use some form of pre-load to ensure that the fine speed of the focuser has enough friction to engage. If it is too tight, the focuser will be stiff, and if it is too loose, it will slip in low speed.
Again, this assumes the the focuser works well in 1:1.
The step relevant to the low speed pre-load is step 11.
Now, if you are using a very heavy load (like Binoviewer or other very heavy load) and the 1:1 knob also slips, that requires that you adjust the tension on the focuser drive shaft support bearing, but again, if the scope is only slipping in low speed and the load is a normal load, you probably need more pre-load.
In some focusers like the Moonlight, there is a spring between the nut and the drive and if this spring is too weak for the load (again, binoviewer is the best example) it sometimes has to be replaced with a heavier spring (the Moonlight Refractor Focusers use a heavier spring than the Newtonian focusers because they have to support the weight of the diagonal when the scope is pointing at zenith. Newtonian rarely gets to a point where the draw tube is vertical).
Anyway, if the focuser appears to work properly in 1:1 and is slipping in 10:1, then I would suggest that there is a pre-load issue.
Edited by Eddgie, 08 December 2018 - 10:45 AM.