Mossberg 500 won't lock up

[colt45cz75]

A friend brought me a jammed up 500 to fix, and I got it apart and reassembled with no problem. However the action does not lock up. Any suggestions before I tell him to send it back to Mossberg? I know I can't let him shoot it like it is. Thanks.

 

[David Henry]

Tighten the forend nut. D.H.

 

[zuke]

No part's missing?

 

[gunrunner100]

Check to make sure the firing pin isn't broken preventing the bolt lock from engaging into the barrel tenon.