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]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "fall behind":
   back, back up, backslide, cock, come after, come last, dawdle,
   drag, fall astern, fall back, follow, get behind, go back,
   go backwards, go behind, hang back, jerk back, lag, lag behind,
   lapse, linger, linger behind, loiter, lose ground, pull back,
   recede, recidivate, regress, relapse, retrocede, retroflex,
   retrograde, retrogress, return, revert, slip back, straggle, trail,
   trail behind