It's not uncommon for them to get sticky and be a bit stubbon when they haven't been used in a long time, or just neglected. It can take several days before grime and dried up oil dissolves and but you can speed it up by adding a drop or two of oil in all places, run the machine, turn/push knobs and levers, an give it a new round of oil and running the machine a bit. From under the top lid and behind the face plate you can gently poke the moving parts involved; it can be the swing arm itself, or some part in relation to the setting knob. By moving parts and observing them you should be able to detect where it sticks. If your model has combinational push-turn knobs, they are known to be a bit stubborn, but it's usually all about cleaning and oiling.