Wreck,
Lots of reasons, soft lug, someone switched a barrel, hot loads (centerfire problem usually), no lube on the pivot wearing away the lug, Factory guy who originally fitted the barrel had his bowling league the night before
. Latch engagement, extractor/ejector position, dirt/oil on the receiver face or latch shelf can also be an issue. Part of owning a Handi I suppose. When I shoot at the range I hold up the gun to the light after loading a cartridge and check for the gap. I ain't the best at glueing shims in place.
When removing metal to fit a barrel that will not close all the way it takes very little removal to get lockup or worst go past lock up. Wear or setback of the lug pivot of only .001 or .002 can easily open up a visible gap. Get a centerfire bottlenecked case Handi that has a great lockup and shoot a upper end load and then check for a gap before you open it up. There will be one. Nature of the best.
Guy at the hardware store wants to know why I keep buying feeler gages.
BB