youve got the locking bolt and fork removed??its corroded then,,try soaking it with wd40 or similar,itll tap out soon enough,maybe,theres a grease point that hasnt been used on your cover for a while,its siezed in there,,patience,,try heating the whole cover in an oven on a low heat,,just hot enough so you cant hold it,,try tapping it then,,the idea is to heat the cover and expand it slightly,leaving the adjuster cooler,and smaller,so not long in the heated oven,count to ten and try again,youll get it out,then wire wheel the hole and the outside of the adjuster,copper grease when it goes back together,,so the next poor guy in thirty years dosent have the same drama!!