Yep, introducing some kind of shock to the area that will break up the deposits often helps. If the bolt is proud of the cover surface you can try tapping that up and down, side to side while occasionally blowing any dust out of the hole. Or drill a small hole in the end of the bolt and use a center punch an hammer to tap the bolt side to side.
Sometimes working dry can help as the deposits don't want to "pack up" as much and can be blown out somewhat.
If that doesn't work, then as Goatracer says, try to get some penetrant in there to try to disolve the deposits.
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