WordNet (r) 3.0 (2006):

fall flat
    v 1: fail utterly; collapse; "The project foundered" [syn: fall
         through, fall flat, founder, flop]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "fall flat":
   bomb, break down, capsize, careen, collapse, come a cropper,
   come to naught, come to nothing, crash, fall, fall dead, fall down,
   fall headlong, fall in, fall over, fall prostrate, fall short,
   fall stillborn, fall through, fizzle out, flounder, flunk,
   get a cropper, get bogged down, get hung up, get mired,
   go to smash, list, lurch, peter out, pitch, poop out, spin, sprawl,
   spread-eagle, stagger, stumble, take a fall, take a flop,
   take a header, take a pratfall, take a spill, tilt, topple,
   topple down, topple over, totter, trip, tumble, turn turtle,
   wash out, wear thin