United India Insurance Company [UIIC] — claim settlement dispute

My vehicle was insured with uiic & went under claim on 12th feb`2019 due to an accident, where survey was done on the spot & at service center too as well by two different surveyors appointed by uiic.
All the damages were endorsed & approved by both the surveyors.
At the time of payment uiic told me that around 60% of total bill would be processed but when i asked the reason of this deduction after approved claim, they made this 60% as 80% but still there is a gap of 20% which they denied to pay even after my several request. Also they are not able to explain as to why this deduction is happening.