Indigo Airlines — not refunding my taxes on flight cancellation

I booked a flight and applied a voucher of ₹1000 provided to me by Indigo in lieu of delay in my previous flight (indigo promise which I purchased) and they compensated the delay of 4hrs with a voucher of ₹1000.
Now I applied this voucher for my current booking, due to some reasons I have to cancel my flight and Indigo is neither refunding me my taxes not providing me voucher that I can reuse for another booking.

My concern is that ok even if they donot return the voucher, they cannot have the Taxes amount.

That amount is paid to govt. The voucher they have provided should be for their airfare and not for the taxes.