|That's almost always due to the cord. The end of the cord where the plug is soldered tends to get the most back-and-forth play that weakens the individual strands. Plus, I believe they use a heat treatment to seal it there and it wouldn't suprise me if that weakens it also.|
You can connect a DC voltmeter to the +/- parts of the plug and see if you get anything.
The only other thing would be if the contacts in the jack in the laptop were flared out and weren't making a good connection.
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