zaostajati

1. to lag behind; to fall behind
   Mali stalno zaostaje za ostalom decom na planinarenju. — The little one keeps falling behind the other kids on hikes.
   Srbija još zaostaje za Evropom po platama. — Serbia still lags behind Europe in wages.
2. (in sports, in score) to trail; to be behind
   Zvezda zaostaje dva gola na poluvremenu. — Zvezda is trailing by two goals at half-time.
3. (of a clock) to run slow
   Moj sat zaostaje pet minuta. — My watch is five minutes slow.

To lag behind someone is “zaostajati za + instr.”: zaostajati za drugima.