fall behind
verb1
Used other than figuratively or idiomatically: see fall, behind.
2
Idiomatic To be late (for a regular event)
- You're falling behind with the rent.
3
To be progressively below average in performance.
- You could lose your job if you keep falling behind with your work.
- It's not like John to fall behind in maths.
4
to fail to keep up (with)