WordNet (r) 3.0 (2006):

fall behind
    v 1: retreat [syn: fall back, lose, drop off, fall
         behind, recede] [ant: advance, gain, gain ground,
         get ahead, make headway, pull ahead, win]
    2: hang (back) or fall (behind) in movement, progress,
       development, etc. [syn: lag, dawdle, fall back, fall
       behind]