SQL Server transaction causes query timeout - sql-server

Question

I have this VBA code in MS Access (Connect, Exec and Disconnect are my own ADO subs/functions):
Private Sub cmdShipOrder_Click()
Dim intShipmentID As Integer
Dim rsShipment, rsShipmentDetail As Recordset
On Error GoTo ErrHandler
If Me.ReservationStatus <> "ZBOŽÍ KOMPLETNĚ REZERVOVÁNO" Then
MsgBox "Nejprve je nutné do objednávky rezervovat hmotné zboží.", vbCritical + vbOKOnly, "Chyba"
Exit Sub
End If
If MsgBox("Bude vytvořena expedice a celá objednávka bude označena jako expedovaná. Pokračovat?", vbExclamation + vbYesNoCancel, "Upozornění") <> vbYes Then Exit Sub
Connect
Exec "INSERT INTO tbl1Shipments (CustomerID, ShippingMethodID, ShipToID, CarrierID, ShipmentCode, DateShipped) " & _
"VALUES (" & Me.CustomerID & ", " & _
Me.ShippingMethodID & ", " & _
Me.ShipToID & ", " & _
Me.CarrierID & ", " & _
"'" & Year(Date) & "EXP" & Format(DCount("*", "dbo_tbl1Shipments", "ShipmentCode LIKE '%" & Year(Date) & "EXP%'") + 1, "000") & "', " & _
"GETDATE())"
intShipmentID = DMax("ShipmentID", "dbo_tbl1Shipments", "CustomerID=" & Me.CustomerID)
Conn.BeginTrans
Set rsShipment = CurrentDb.OpenRecordset("SELECT * FROM dbo_v_SalesOrderSub WHERE SalesOrderID=" & Me.SalesOrderID)
rsShipment.MoveFirst
Do Until rsShipment.BOF Or rsShipment.EOF
Exec "INSERT INTO tbl1ShipmentDetails (ShipmentID, SalesOrderDetailID, Quantity) " & _
"VALUES (" & intShipmentID & ", " & _
rsShipment("SalesOrderDetailID") & ", " & _
rsShipment("Quantity") & ")"
rsShipment.MoveNext
Loop
Set rsShipmentDetail = CurrentDb.OpenRecordset("SELECT * FROM dbo_v_ShipmentSub WHERE ShipmentID=" & intShipmentID)
rsShipmentDetail.MoveFirst
Do Until rsShipmentDetail.BOF Or rsShipmentDetail.EOF
If rsShipmentDetail("ProductTypeID") <> 3 Then
Exec "UPDATE tbl1Units " & _
"SET ShipmentDetailID=" & rsShipmentDetail("ShipmentDetailID") & " " & _
"WHERE SalesOrderDetailID=" & rsShipmentDetail("SalesOrderDetailID")
End If
rsShipmentDetail.MoveNext
Loop
Conn.CommitTrans
rsShipment.Close
Set rsShipment = Nothing
rsShipmentDetail.Close
Set rsShipmentDetail = Nothing
Disconnect
Exit Sub
ErrHandler:
MsgBox "CHYBA: " & Err.Description
Conn.RollbackTrans
Exec "DELETE FROM tbl1Shipments WHERE ShipmentID=" & intShipmentID
If Not rsShipment Is Nothing Then
rsShipment.Close
Set rsShipment = Nothing
End If
If Not rsShipmentDetail Is Nothing Then
rsShipmentDetail.Close
Set rsShipmentDetail = Nothing
End If
Disconnect
When I run this without Conn.BeginTrans and Conn.CommitTrans, the code works just fine. However when I leave the transaction commands there, I get a "query timeout" error from SQL Server. Specifically on a second cycle in the INSERT INTO tbl1ShipmentDetails statement. Why?