SEATTLE (AP) — Recent mass shootings have spurred Congress to try to improve the nation's gun background check system that has failed on numerous occasions to keep weapons out of the hands of dangerous people.

The problem with the legislation, experts say, is that it only works if federal agencies, the military, states, courts and local law enforcement do a better job of sharing information with the background check system — and they have a poor track record in doing so. Some of the nation's most horrific mass shootings have revealed major holes in the database reporting system, including mass...