fall behind in (something)

1. To not be as current with a task or activity as one should or ideally would be. I fell behind in my schoolwork when I had the flu. I've fallen behind in this season—can you tell me what happened in the latest episode of the show? A: "Did the due date get moved up or something?" B: "No, I just fell behind in my timeline when I was out for a week on vacation."

2. To move or act more slowly than someone else. I fell behind her early in the race and could just never catch up. A: "I didn't expect you to fall behind in the race right from the get-go." B: "Yeah, well, that's what happens when your car stalls at the starting line." I'll stay at the back of the line to corral anyone who falls behind in our walk to the park

3. To not be up to date on one's current financial obligations. All of those medical bills have caused me to fall behind in the rent. A: "If you'd had more money saved up, you wouldn't have fallen behind in your bills. Just sayin'." B: "Gee, thanks." Because I'm still waiting for three clients to pay me, now I'm falling behind in paying my own bills.