fall behind with something

Từ điển Oxford Advanced Learner 8th

ˌfall beˈhind with sth derived
(also ˌfall beˈhind on sth especially in NAmE)to not pay or do sth at the right time
•They had fallen behind on their mortgage repayments.
•He's fallen behind with his school work again.
Main entry: ↑fall_derived