Life insurance claim rejected - incomplete documentation

My father passed away 2 months ago due to heart attack. He had a term insurance policy of 75 lakhs crores from HDFC Life which was bought 6 years ago. When we filed the death claim, the insurance company rejected it saying incomplete documentation. We have submitted all documents including death certificate, policy papers, hospital records, and FIR. The insurance company is not giving a proper explanation. We are a middle-class family and this claim amount is very important for us. What can we do now? Can we file a complaint with IRDAI? Should we hire a lawyer? Has anyone successfully challenged a claim rejection? Please guide us.