Often the washer will slide off slightly as you try to screw the stop motion knob back on and it isn't always obvious which makes it not seated correctly and won't work. You could try sitting the machine up on its nose (held upright with one hand) as you place washer and screw knob back on. Of course, that only works if it's not installed in a cabinet. Alternatively, try holding the washer in place with the tip of a flathead screwdriver as you screw knob down.
Did it work before disassembly? I have a Kenmore whose stopmotion wouldn't work but it was a stuck secondary pulley and it worked after cleaning that...so there are some exceptions.