Nagendra,
The issue is the system is not doing a collective check instead it is doing one by one even though firming of planned order is set
Well, when the system does a collective check, it always does each check one-by-one. This is normal. 'Firming' is irrelevant to ATP. Firming is only used for planning, not for ATP.
During ATP, each requirement being checked 'competes' for any existing availability. Availability is computed by including all the supplies and all the demands that you have placed in your scope of check.
Your problem seems to be that the system is not respecting the results of the checks for any other planned orders that have already been confirmed. In order for ATP algorithm to 'know' that other planned orders have already been checked, and that they should reduce the ATP quantities for the CURRENT order being checked, you have to check "Incl. dependent reqs". If you think you will also be converting these to production orders, you may also want to place either an 'X' or 'A' in "Incl.depend.reserv" as well.
CO09 shows only the supply and demand elements that are being checked in the ZB rule. Since you are checking no other requirements, no requirements are displayed. What is a bit puzzling is that you said there was 200 in stock of B; but your CO09 says zero. Can you display an MMBE of B?
Best Regards,
DB49