Full-duplexing / fiber and Transceivers

A student of mine has a problem with a network he is administering. He has fiber running between sites with Garrett transceivers (10/100)on both ends. The transceivers will not auto-negotiate (will not advertise full-duplex), and he has to switch tohalf-duplex for it to work. Any thoughts as to why the transceivers are being recalcitrant? He has talked to support at Garrett and their comment was “Well, it should be working.”