That little horizontal pin is easy to miss and hard to locate the first time - it throws everything off if it has one little teeny tiny dried up piece of oil - I clean it first thing when I do the top of the machine - q-tip and solvent. Protect the area with cosmetic pads or panty liner pads to absorb solvent - also swab out with a q-tip when you are done - I use forceps to get the q-tip down in there... and forceps to get the pads out. After it has been clean you will have to re-oil it. The knob will not turn if those posts with all the slots are gunked up - some times it it the under side of the thingy that rests in the notches. Pushing in or pulling out pulls that little thingy out of the grooves. If there is gunk you won't be able to work the knobs. You can't see that dried up oil - even a tiny drop will keep it from moving. There is a reason the repair man wants $179 to do this kind of clean up job. Some times it takes hours. Some times it takes days. You have to be very thorough and even then some times you miss something. A few weeks ago I missed something dried up in the gears under the bobbin area somewhere. It doesn't take much dried up oil to stop up one of those Singer 401 machines.
BTW I run my 403 all the time with out the cam on there. Never had a problem - I don't get it I guess. I think it is the Singer 328, 319 etc - some other old machines you have to worry about.