To produce no effect; to fail in the intended effect.
The politician's speech fell flat.
Lord Erskine
Of all who fell by saber or by shot, / Not one fell half so flat as Walter Scott.
2012 May 13,Alistair Magowan,“Sunderland 0-1 Man Utd”, inBBC Sport:
United were made to pay for letting an eight-point lead as little as five weeks ago slip and, despite their final-day efforts, they will look back on a season where, when they needed to step up, they fell flat.