Problem freeing memory

Hello.
I have problems freeing resources on my application. I don't know if I'm
doing the things well, so please tell me where could be my mistake.
I have some pointers (structures) that I store on a Tlist. Some of that
structures have lists that store more pointers.
So I have something like this:

  TAAmostra=^TAmostra;
  TAmostra = record
    NumAmostra:integer;
    N:integer;
    hora:TDateTime;
    ListaPesagens:TList;
    ...
    prod:TPProduto;
    ...
  end;

  TPPesagem=^TPesagem;
  TPesagem = record
    N:integer;
    MBRUTA:single;
    Tara:single;
    MLiquida:single;
    VLiquido:single;
  end;

In this situation, the structure TAAmostra has a list (ListaPesagens) that
is going to store the TPPesagem structure.
So then I have this list:

listaAmostras:Tlist;

and the variables

amo:TAAmostra;
pes:TPPesagem

I then create a new dynamic variable and place it in the TList:

        new(amo);
        amo.numamostra:=b;
        ...
        for a:=1 to 5 do
        begin
            new(pes);
            pes.n:=a;
            ...
        end;

        amo.listapesagens.add(pes);

        listaamostras.add(amo);

So, in this way I can easily access the info I want like this:

procedure search(amo:TAAmostra);
var
    a:integer;
    pes:TPPesagem;
begin
    for a:=0 to amo.listapesagens.count-1 do
    begin
        pes:=amo.listapesagens.items[a];
        ...
        ...
    end;
end;

So, this is how I manipulate the structures.
To free them I do something like this:

        for a:=0 to listaAmostras.count-1 do
        begin
            amo:=listaAmostras.Items[a];
            for b:=0 to amo.ListaPesagens.count-1 do
            begin
                pes:=amo.ListaPesagens.items[b];
                dispose(pes);
            end;
            amo.ListaPesagens.Clear;
            amo.ListaPesagens.capacity:=0;
            dispose(amo);
        end;
        listaamostras.clear;
        listaamostras.capacity:=0;

My problem is this: I have a procedure that "refreshes" the data in the
structures. To do so, I want to free the memory allocated to the TAAmostra
structure and then create a new one with the new values. Well, it seems it
isn't freeing the memory, because if I run that procedure 20 times in a row
I get an EoutofMemory exception. But if I'm freeing the memory and then
allocating space for exactly the same data, the occupied memory should
always be the same!!!
I know this description is a little long, but this is very important. My
deadline is next wednesday and I must have this problem solved. Anyone can
help?
Thanks for your time.
Best Regards

Goncalo Martins