fall behind

be backward, be slow, go down, deteriorate (in one's studies, etc.)

was behind everyone else, lagged behind all the others, stayed in the back

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Verb
1. retreat
(synonym) fall back, lose, drop off, recede
(hypernym) regress, retrograde, retrogress
2. hang (back) or fall (behind) in movement, progress, development, etc.
(synonym) lag, dawdle, fall back
(hypernym) follow
(hyponym) drag, trail, get behind, hang back, drop behind

be unable to run as fast, drop behind You will fall behind in your work if you miss a day of school.

ming2 luo4 sun1 shan1
Falling behind Sun Shan.
To fail in a competitive examination or a contest.

Synonyms and related words:
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