Delphi 5 Ent - Windows 2000

Hello delphi's world,
 I have an ambiguos problem and i'm not able to understand the cause it
Take a look to the piece of code under here. I just make some
elaborations on a TList
object.  At every iteration, for time-consuming reason, i call the
ProcessMessage method
of the main Application object.

This code is the click event handler of a button on the MainForm.

I := 0;
   bEsciCiclo := False;
   While (I <= ListSchiera.Count - 1) and (Not bEsciCiclo) do
      ItemSchiera := ListSchiera.Items[I];
      if (ItemSchiera.dSched_DataOra <> dDataZero) or
        (ItemSchiera.dSched_Part - GL_Inizio_AK_H > Now) then
         bEsciCiclo := True
         NextOperation(ItemSchiera); {Nel caso di anagrafica ho generato
AK ed EP}
         ListSchiera.Items[I] := Nil;
   {Ordino la schiera}

The problem is:

     It doesn't come back after the call to  Application.ProcessMessages

so it doesn't do the Pack and Sort calls.
The same code on a  Windows NT 4 service pack 6 - Delphi 5 Ent. system
goes rigth.

    Grazie anticipatamente per la vostra attenzione