Airlines not giving claim on lost bags what to do

Dear Sir/Madam, We have lost our bag on 2nd July 2016 during travel from Singapore to Kolkata via Dhaka. My Wife and I regularly communicated with Biman Bangladesh Airlines stuff in Kolkata/Dhaka/Singapore for Bag status. Initially they provided hope that bag will be found and may be in Dhaka airport lost during transit. Now after so many days they have provided the claim form and we have filled up the form with all required documents. Biman Bangladesh Airlines has agreed to give us USD 140.00 compensation for the bag loss which we have not agreed to receive as we are not satisfied with this value compared to what we have lost and also because we have travel insurance which can provide better compensation. Now Biman Bangladesh Airlines stuff says they have close the case because we have not agreed to receive their compensation value. My Insurance company asked to get a report from Biman Bangladesh Airlines. They are not providing that, not even a response anymore. Pls help us.