DataTable delete row does not take effect

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VB

Hi experts,
I hope you can help me.
I have 12 datatables in my VB.net code.
I have to add in the first table on an event a recordset. It works.

If the number of recordsets in the first table is above 60, I have to take the oldest recordset and transfer it to the second table. If the second table contains after this more than 60, I have to transfer again the oldest one into the next table.

Do Until QuellTabelle.Rows.Count = Pulse

            Dim DT As DataRow = ZielTabelle.NewRow
            'Debug.Print(QuellTabelle.Rows(0).Item(1))
            Dim ODR As Integer

            For ODR = 0 To ZielCol Step 1 
                DT.Item(ODR) = QuellTabelle.Rows(0).Item(ODR)
            Next ODR

            DT(Präfix_Ziel & "_Start") = CDate(Now) 
            DT(Präfix_Ziel & "_Temp") = CDbl(QuellTabelle.Rows(0).Item(Präfix_Quelle & "_Temp"))
            ZielTabelle.Rows.Add(DT)

            QuellTabelle.Rows(0).Delete()

        Loop

It works for the transfer from the first one to the second one really good. But from the second to the third I always get an error:
The problem is, that the QuellTabelle.Rows.Count does not change. And I do not understand why. What do I have to do after the delete command?

Thanks in advance!