As Eric points out, a large portion of blame rests with the licensing agencies, who are understaffed to handle the large number of small requests that come in. When I was in charge of reconciling the mechanical licenses for a local community chorus, it was impossible for us to find out which agency to pay for about 20% of the back royalties we owed. AFAIK, that money is still sitting in the escrow account we set up in case anyone ever showed up to ask for it.