Probably never lubed and gummed up with road dirt and seized, a generous squirt of Plud Gas or other proper penetrating oil will most likely free it off enough with a bit of jiggling, grease and oil the whole release mechanism once you get it open and the safety catch too.

Our Fiesta pool car the mechanism was seized too, again lack of a bit of oil/grease when serviced/looked at whilst having a fag.

See also: www.youtube.com/watch?v=JDmdYXJzOAo