About a week ago I was out taking some photos with several cameras at a connservation area, a small one for wetland birds, a small marsh areas with some trails and some trees but mostly open. Temperature was about +3 C, no wind, and sunny. I was using a Kiev 3 with Jupiter 12, a Nikon 5700 and a Zorki 4 with Jupiter 8 50mm f2 lens and Jupiter 9 85mm f2. I like the Zorki viewfinder and I was trying to determine for once and for all if the Jupiter 9 focused correctly. About ten shots into the roll of Superia 200 24X, the Zorki shutter will not fire. This particualar example I had tested simply and had thought that the shutter seemed to be working fine at shutter speeds above 1/60th. 1/60th would hang, slowly closing the second curtain. When I was shooting at the marsh, I was shooting 1/250th f11 and maybe f16. Took about 7 or 8 shots and then switched to the Jupiter 9 lens and after three photos, the shutter refused to work. I had not changed shutter speeds. I have learned not to change shutter speeds on these Soviets until I have advanced the film and recocked the shutter. I gave up on it and used the other cameras. At home I rewound the film and then tried the shutter again, worked like a charm. Any thoughts as to what the heck went wrong. I thought I might ask before trying another roll of film through this camera. Thanks for any thoughts.