Need Guidance

Hi everyone,
I’m looking for some guidance from people who got a B1/B2 visa approved after multiple rejections.
My father applied for a U.S. visitor visa twice. Last year in March, he applied as self-sponsored and was rejected. This year, I sponsored/invited him as his daughter. His interview was at the Delhi Consulate.
This time, the officer asked only two questions:
What does your daughter do in the USA?
What does your son-in-law do in the USA?
After that, his visa was rejected again. No documents were checked.
Has anyone faced a similar situation where parents were rejected even after sponsorship from children in the U.S.? What did you change before reapplying, and what helped you get approval later?
Any advice on what we should improve in his DS-160, interview answers, purpose of visit, or supporting documents would be really appreciated.
Thank you.