The answer to 'P-trap not working' is that the P-trap, the curved pipe under your sink, may be clogged, leaking, or not installed properly. Common symptoms include water backing up in the sink, foul odors, or even water leaking from under the sink.
Some possible solutions include clearing a clog, tightening the P-trap connections, or replacing the P-trap entirely. For minor issues, a homeowner may be able to fix this themselves, but for more complex problems, it's best to call a professional plumber to diagnose and repair the issue.