Here's an edited version of your photo to show what I suggest you do. I think Kenneth Elwell has it right, and that it has little to do with the adjusting arm.
There might be a lot of gunk preventing the free movement of the opposite cutting blade. If you remove that blade, clean and lube its bolt-hole, and then reassemble you may likely have more success.

"We carry a new world here, in our hearts..." --Buenaventura Durruti
"Don't wish it were easier. Instead, wish you were better." --Jim Rohn