How can I obtain a clear title for my property to purchase title insurance?

My father purchased a vacant lot in 2005 and had a warranty deed signed, notarized and filed with our county. My father let me have the property to build a new home, once the home was completed I had the deed transferred in my name, now I am know trying to obtain a equity-loan to pay for the loan I had previously taken out to build the homebut my problem is the owner from which my father bought the property from was one of 3 owners the property was willed to from back in the 70's. Is this a pretty common issue? I already have a title company I am working with, but they want me to get an special warranty deed signed by the two other deceased owners heirs, but they do not reply to any letters I mail them? What can or should I do? I really need to obtain the loan, but my lender will not approve it without the title insurance.