sales order stock

Hi ,

stuck with sales order issue  ,

when i create sales order for xyz customer for a A item with  perticular qty , order gets save ,

but when i create second order for same xyz cutomer with same A material with  different qty . and when i try to deliver the second order , message received " No schedule line is due for delivery ",   I have a stock for this A material ,  my question is is the qty gets reserved for first order ?

this type of requirement is not required for client, please help to get resolve this issue

Thanks

sourabh