Purchaser will not comply with contract and ignores my requests

Im not certain how to make a purchaser follow the sales contract on property that I am selling to them.  The real estate attorney who drew up the contract for the sale included that the buyer had to acquire insurance on the property and keep it until the final payment was made.  It has been over a year now and the purchaer ignores my request for the proof of insurance.  The attorney was very expensive while drawing up the contract and purchaser continued to question if I was the owner, therefore affidavits of heirship had to be provided which cost even more.  The attorney who drew up the contract said he would write them a threatening letter for 300.00 asking for the proof of insurance, but if they continue to ignore the request I will be out even more legal fees.  Do I just let it go and stop asking them to follow the contract?  I am afraid I may be liable if something happens and the property is not insured. Any advice would be appreciated?