Could Not Find Record error

I'm using an Access database with Delphi front end.
I'm using ODBC to gain access to my Access database.
I have the following code,

   If tblDMC.Locate('CSDB;DMC_IDX',
VarArrayOf([DMAcquired.tblDMCISSNOCSDB.AsVariant,
DMAcquired.tblDMCISSNODMC_IDX.AsVariant]), [loCaseInsensitive]) Then
   Begin
     tblDMC.Edit;
       tblDMCExcluded.AsBoolean := True;
     Try
       tblDMC.Post;
     Except
      On E : Exception Do
       MessageDlg(E.Message, mtError, [mbOK], 0);
      End;
   End;

The Locate method returns true always (as the record is selected by the
user)
When the code gets to the post an exception is raised and the message

"Could not find record"

I'm struggling with this, any opinions?

Thanks.

Email: j.demp...@LogisticsBusinessSystems.Ltd.Uk