I'd soak the front part of the plane in WD-40 or something like that, or really put a lot of the same thing on the front where the adjustable plate is.
After it soaked, I'd loosen the front knob a few turns and then tap on the knob with a soft mallet (like wood or plastic) to see if that would break it free. If it does, then you need to clean the metal on both the body and the sliding throat plate.
About all I can think of.
Go into the world and do well. But more importantly, go into the world and do good.