The symptom fits for either plate getting stuck, or spring getting old-
Between the two- if the plate slides freely, the spring is the more likely culprit.
They sell a refubishment kit that includes replacement parts (chain, spring etc)
here is list MSRP is 5.99
http://www.industrialrev.com/refurbishment-kit.htmlfound actual kit searching for UCO Refurb Kit- amazon 9.68