there were several types of 3,5 F cameras. The first model had something like a EV-locking device between both wheels and a concentric "key" to unlock this lock amd turn the other wheel. The customers were not happy with this (I can understand this becuse I have one) and then Rollei introduced a little differential gear and both wheel can be turned independently. But compared to an older Flex without cross-coupled exposure meter you have to turn with a little more power. Maybe the lubricants of your camera are gummed and you should invest in a CLA.