Roofing nailer not advancing nails.?

Roofing nailer not advancing nails.?

I have a bostitch n45b roofing nailer and it will not advance nails. It will shoot one nail but won't advance the next one into the firing position. Have to open the gun up and move the nails and shoot again and same thing. What might be causing this problem? I'm not sure how the pawl works so don't know what to look at. I tried searching on the web because I thought this might be a common problem but didn't find much. Thanks for any help you might be able to share on why the nails aren't advancing.

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