Sometimes this shutter remains blocked because of a broken very small spring behind the command plate on the right side of the camera.
In http://digilander.libero.it/foto_ras/FP_shutter_SpeedGraphic.htm (second part of the page) you can see how I operated for this case.
Sorry, it's in italian and Google translation is (as I can see) quite ridiculous, but maybe the photos are anyways useful.
Hoping this can help some beginner in LF.
Thanks for your attention.
foto_ras