Sum calculation doesn't return value unless there is more than 1 entry

[JackieGeek]

I set up an order form with automatic calculations. The calculations are working line by line, but the subtotal at the bottom only returns a value if there is more than one entry on the order. I have double checked that I included all the fields I want summed, but it doesn't return anything unless there is more than one quantity entered on the order (so two or more totals to sum). If there is more than 1 total to sum, it picks up all the proper fields and returns the correct value.
Can I fix this so it picks up even if only one quantity is entered?
I have the document on our website if needed - see URL below.