Sorry, forgot you had a commercial model. I don’t want you to take it apart, could be a disaster if you are not mechanically inclined. Try this first. Get a can of a good spray penetrating oil. Spray down the area around the adjuster arm and let sit for 30 minutes. Come back and spray again, then start working the adjuster arm back and forth. With any luck you should be able to free it up. Once freed up spray again and let it sit for 24 hours and repeat if you have to. Let us know what happens.