Material Determination - ATP Issue

Dear Experts, We are using material determination in our project. The reason is 0006, if material A is out of stock, system will generate another item with material B to substitute mate rial A. For e.g. customer order material A 10 PC, since there is no stock, system will substitute material A with material B. Here in sales order it is showing: Item Number item Category Material Number Quantity ATP 10 TAPA A 10 20 TAN B 10 9 Once delivery note created for this sales order wiht 9 PC, status of item 10 goes to complete, 20 goes to partially delivered. However, after receiving 10 PC of material B into stock, nothing happened when execute ATP to item 20 or even item 10. There is no schedule line generated for item 20. Moreover, all the items became grey out in sales order. Item 20 is always open and could not be confirmed or shipped even if we have sufficient stock. Has anyone encountered this situation? Any idea will be very helpful. Thanks! Regards, Lu Lu