In the case of gun control legislation, how can an interest group like the National Rifle Association (NRA) be successful in blocking legislation despite bipartisan support?

As an interest group, the NRA is a group of people who share the same value (for example, the right to bear arms). To protect these shared interests, an interest group can hire lobbyists, or professionals who work with government officials to protect the shared interests of this group. Since the NRA spends quite a bit of money on lobbying ($3,360,000 in 2014), they have a huge influence on the government, and can thus block legislation despite bipartisan support.