'fall behind' definitions:

Definition of 'fall behind'

From: WordNet
verb
Retreat [syn: fall back, lose, drop off, fall behind, recede] [ant: advance, gain, gain ground, get ahead, make headway, pull ahead, win]
verb
Hang (back) or fall (behind) in movement, progress, development, etc. [syn: lag, dawdle, fall back, fall behind]

Synonyms of 'fall behind'

From: Moby Thesaurus