My newly acquired Rolleiflex 2.8C appears to have a problem with changing shutter speeds. I did some testing with some of the faster speeds and all was well. I then switched to "B" to check out the lens glass; no problem there either. But now it won't let me move the speed dial past 1/5 second to get back to the faster settings. I feel it clicking, as if gears are slipping past each other instead of being properly engaged. Have I missed something with the speed & aperture controls, or is this a repair issue? Thanks for any help.