Best reply to a scenario in Workplace

I have scenario where I was supposed to send a report to a manager every week. I wasn't able to prepare the report for a given week and when asked by my lead I told that I couldn't do but will send from next week onwards.
Instead of letting it go he kept asking why weren't you able to do .
I can't give any particular reason for not doing and also instead of just repeating "I couldn't do" , i want to give some diplomatic answer

What should be my reply to this question?