Forms :: Form Load Calculation Not Working

I have a bound form to an invoices table. The form details all invoices raised, all payments made and an outstanding balance. The outstanding balance control is just a basic text box.

On form load, I go to the payment table and select all payments into a recordset, I then add up all payments and use the following calculation:-

Balance = InvTotal - paytotal

The Invtotal is the control on the form that has it's control source set as sum(invoicetotal)

The calculation used to work and the correct balance was displayed, indeed, for one particular client, the calculation works, but for all others, the summed value is blank and so the balance is effectively set to the paytotal value.

I know this is to do with the timing as the form loads and it's records/data are made available, as I have an update button that does exactly the same things as the form load and when pressed, all numbers work.....primarily because the summed invoicetotal value is now available in the form control.

I can see what is happening, but not sure why...I thought that form load gave you all records and would have populated the summed control before triggering the load event....obviously not.....