There are three cams - A B and C - IIRC correctly, there are four stitches plus zigzag on each cam, and you DO need a cam to perform a zig zag. Cams are about impossible to find without a machine, I've been looking for over a year now.
Check out the cams to make sure the tabs are not broken off.
Top thread guide breaks off on these easily
They are cast iron, so HEAVY machines.
If you need parts - you may get lucky if someone has them on eBay - or you buy a donor machine. We had four machines to get two fully functioning. And then easily sold remaining parts for good money. Also - there are many variations of the 21, we have an A and E, and many parts are not interchangeable. In fact, even if you have a older and newer 21A - some parts are different.
They are a very nice sewing machine, all metal gears and though not a lot wears out, the tension knob seems to be in a location where it gets easily broken. We sold one tensioner for $50 and they guy was every so happy to get it too.