Drop-off and Fall behind

Drop-off noun - The amount by which something is lessened.

Fall behind and drop-off are semantically related. In some cases you can use "Fall behind" instead a verb phrase "Drop-off".

Fall behind verb - Hang (back) or fall (behind) in movement, progress, development, etc..

Drop-off and fall behind are semantically related. Sometimes you can replace phrase "Drop-off" with "Fall behind", this terms are similar.