British Mark IXA Bubble Sextant

[LoneStarGazers]

Does anyone have experience with bubble sextants?  I recently acquired one, but am finding it difficult to move the Slow Motion Knob.  When I first inspected it, the knob seemed to moved freely.  Then I mistakenly turned the winding crank only one click.  After it wound down, the knob was extremely hard to turn, almost as if it is locked so the user can get a reading?  Maybe there is a way to unlock it?  

Any help is greatly appreciated!

Mike