To fall behind is to fail to keep up with others or with payments. Fall behind with the rent, fall behind the rest, do not fall behind.
The money sense is the most serious, and falling behind with rent has a legal meaning of its own. It takes with before payments and behind before people.
1phrasefall behindTo fail to keep up with the pace of others or payments.
“Miss two months and you fall behind in a way that is hard to fix.”
fall behind is a phrase.
Also accepted in games: lag behind
Practised in Phrasal Verbs