ATP Available to Promise checking

Question

Hi SDNrs,
ATP appears to not be working... at least not the way we want it to.  
We have a stand alone warehouse that is a separate plant in SAP, WH gets materials from a separate manufacturing plant linked thru the special procurement key.  MRP works fine but ATP checking in the warehouse does not look at the actual production orders in the separate manufacturing plant, so if production order dates or qtys are changed, ATP does not see them and ATP appears to be only checking in the sales order supplying plant - the warehouse in our case.
We are considering creating user exits and much custom code to change ATP - not usually a good plan!.
Is there a way to config SAP so ATP looks thru the warehouse into the manufacturing plant to see the actual production orders?? 
We need to supply customers with accurate promise dates based on any production order changes in qtys or dates....
Please help as the custon code is a real big project.
Regards,
Paul

Answer 1

Thanks for your time Suda,
I created the same query yesterday without success, but I tried yours amnyway. Who knows with computers hey
But no, it is still not solved. My available quantity is 10 in WHS 01. When crating the SO for 11, it gives me the error, when asking for 10, it gives me the error, and for 9 and 8. But for 3 or 2 units, it goes through.
I created a work around yesterday as I had to deliver to the client, but the work around is extremely sensible to time of adding the Sales order (as I use a FS for calculating the ATP and use the ATP field in the same store procedure -> That's work)
You see, the Store Procedure works 100% when comparing to a value (>5, or <=4 etc.) but not with a calculation within two field like Onhand-Commited!!!!
So I still need that query to work and I would appreciate if you can continue your investigation.
Kind regards,
Frederic