I also have an EX21. It is a beautiful machine.
Be aware of how the detent works to hold the arm up. It is only about 1/2 turn from no engagement to full. If you turn it in too far, it will hold up the arm by contact friction rather than the spring ball engaging. The friction will gall the internal surface that the ball is supposed to fall into.
Also make sure the blade is centered in the through hole in the plate. If it is not centered when you get it, you can loosen the 4 flat head cap screws that hold the plate to the frame and move the plate so the hole is centered on the blade.
If you didn't yet, buy a foot pedal and a light. They make the work 100% more enjoyable.