I just got a new 18.5 in barrel for my 870 (Mossberg made for an 870), but the slide will not slide all the way up (and the bolt doesn't fully engage). There's about a 1/4" of travel left on the slide and the bolt is almost in position. The new barrel looks identical to the old one and it worked fine. I can't see anything problem and the internal alignment looks to be identical to the old barrel. Does anyone have any ideas?