Not familiar with that machine but stuck attachment wedges are fairly common. The lever will move but the wedge is stuck.
The most common problem is when an attachment is left on for years without any wedge movement. They rust and or seize. I always move the wedges up and down every time I run a machine as a maintenance measure.
They normally have grease points that get missed and are just neglected.
You can try soaking with penetrating oil. There are also tools to force grease or oil into the grease zerks.
For the most stubborn wedges I have had to use a sledge hammer by raising the arms up far enough to be able to hit the wedge from bellow. This is dangerous and could damage the wedge.