Does that whole white cartridge thing rotate inside the fixture? If so, it looks like that metal tab is supposed to cover/uncover the hole in the fixture to control water flow when the handle is rotated. And the black rubber seal thing must ride on the bottom of the metal tab. If so, then I'd suspect poor fit, too short, or bad rubber seal, or lack of spring tension/preload. If that whole thing does NOT rotate, then I'm stumped.
Disclaimer: I an't no plumber.