Turn dials are notorious for getting stuck, especially the more fancy ones, some are literally geared, others are combo push-turn things. The simpler ones are usually easier to unstick. Either way, it's usually all about getting a degreaser, solvent or spray lubricant to the inners of them. Ideally some should be picked appart, cleaned and put back together again, but I never feel safe unless I have a very precise guide on it. You are spot on being careful with plastic knobs.
I just read something on the 6000 series; knobs and cams can be difficult. An expert on this brand insist parts have to be "rebuilt", and perferably by an expert. There's hardly anything on the 5200 numbers, but nothing is impossible. A thin spray can oil with teflon?