1.
If you fall behind , you do not make progress or move forward as fast as other people.
Evans had rheumatic fever, missed school and fell behind...
Boris is falling behind all the top players.
PHRASAL VERB : V P , V P n
2.
If you fall behind with something or let it fall behind , you do not do it or produce it when you should, according to an agreement or schedule.
He faces losing his home after falling behind with the payments...
Thousands of people could die because the relief effort has fallen so far behind...
Construction work fell behind schedule.
PHRASAL VERB : V P with n , V P , V P n