When the camera's empty with the back open, do you see anything when you release the shutter and advance imaginary film thru it? Just wondering if it's feeding film properly or if it might be the shutter.
I have a Contaflex. It jammed, and some time later when I got it out again it had unjammed itself; I think tension released over time. They're supposedly temperamental, I got mine at an area camera store going out of business but haven't tried using it in a long time.
I've found with old cameras they work better on medium to fast shutter speeds. Trying a slow speed usually results in the camera just sitting there stuck, or the timer slowly winding down and maybe or maybe not actually taking a picture. I can usually hear if it sounds like it's firing fairly quickly or not although I can't tell how close it is to accurate shutter speed. I assume it probably isn't all that accurate so I usually bracket shots.