scheduling order for non atp item

hi ,

i have an order in which there is an item on line that is non atp -- set up at item level for inventory organization. for one line of order , the given request date was 25-jun and it gave a schedule ship date of 18 jun. this line was closed now i added another line , for same request date - 25-jun. but now it is giving schedule ship date as 25 jun only. why is this happening. is there a program that needs to be run. how is schedule ship date calculated for non atp items ? item