Fall behind with
Definition 1 of 2
Pro Tip 1/3Progress Related
Used when someone is not advancing at an expected pace in a specific area. 
Without practice, you might fall behind with your piano lessons.
Pro Tip 2/3Comparison Implied
Implies a comparison to others' progress or to an established standard. 
He's falling behind with his training compared to the rest of the team.
Pro Tip 3/3Responsibility
Highlights a personal responsibility towards a task or duty. 
If you fall behind with your duties, the entire project could be delayed.