Assignment of Invoice number based on line type

Hi,

We have a Sales order with three lines. two with Line type S, one with Line type P(Price Discount).

When user executed the invoice program first it generated invoice number for line type P and did not generate the same invoice number for all the three lines.

So the user had to execute the invoice program again to generate the invoice number again for S type Lines.

My question is: is this a general phenomenon in JDE, If yes, which set-up controls this invoice number assignment.