Screen Door Pneumatic Tube stopper: What are they called and why is mine not working?
They're called hold-open washers. 
It won't be easy to assume based on your description. Perhaps post a photo of where the washer normally meets the metal?


It sounds like the washer is probably just bent. Maybe you can take it off and straighten it in a vice and try it again. If it comes down to it, and you want to save some money.. you can back the set screw out of a drill stop and put a thumb screw in, grind the lugs off the shaft and slide it on!