From The Atlantic:

Republican leaders subjected each of the four amendments to 60-vote threshold and rallied their members around proposals backed by the NRA.

and:

Democratic proposals that would have expanded background checks at gun shows and blocked gun sales to individuals on a terrorist watch list each fell short of the 60 votes needed to advance. Republicans largely rejected the measures and instead voted for alternative bills that Democrats said would be ineffective—one designed to improve, but not expand, the existing background-check system and another that would require the FBI to seek a court order before stopping a gun sale to a suspected terrorist.

And most telling:

The Republican proposals each received 53 votes, while 44 senators voted for the Democratic amendment to expand background checks and 47 voted for the Democrats’ proposal to block suspects on the watch list from buying guns.