Locate error on ADOTable with Master & Detail

Hi,
  I have a problem when using adotable with master datasource. When I locate
a record within detail table using Locate, there is only the first time it
can be correctly. If change the master record, it can not be located
correctly(result:=false, but the record is already shown at dbgrid). why?
please help me?
  Thanks a lot

  tony