fall behind (somebody/something) phrasal verb ( see also ↑ fall )
1 . to go more slowly than other people so that they gradually move further ahead of you:
His mother was chatting and didn’t notice that he had fallen behind.
She hurt her ankle and had fallen behind the others.
2 . to become less successful than other people, companies, countries etc:
After her time in hospital, Jenny’s parents are afraid she has fallen behind educationally.
Companies that are not market-driven risk falling behind the competition.
3 . to fail to finish a piece of work or pay someone money that you owe them at the right time
fall behind (somebody/something) with/on
After losing his job, he fell behind with his mortgage payments.
The project has fallen behind schedule.