Take the bolthead off completely and THEN try to close the bolt. If it does close, then the bolt head or it's geometry is the problem. I don't want to sound facetious or a smart arse, but it's problem solving by deduction. And that's usually the trait of garage mechanics. They'll start by changing parts until it's fixed. But they YOU have to pay for all of the parts, even though some or most of the parts they've replaced weren't faulty in the first place! Oh well.............
When we were apprentices, we all used to moan and groan about why we had to learn, in almost intimate detail, the smallest why's and wherefores of every little mechanical sub-action of the various parts of the weapon that were acting simultaneously, spring action and spring load. But the instructors would throw in low ballers, as if asking an 'experienced' Armourer a technical question over the phone, from 200 miles away (they were still in the classroom of course, with the .300" M1919 sat on their lap while you tried to bull-shi......er...., waffle your way through their problem.
The problem - as any doctor will tell you (and how Roger Payne will be nodding in agreement here......) is describing the problem!