Hi SL,
You will need to enter a strategy group at the plant A. Depening on if you have make to order or make to stock or any other planning mode, you will need to enter strategy 40, 10, 20, etc.. You can read about it in the SAP help:
Planning Strategies - Demand Management (PP-MP-DEM) - SAP Library
Now, in your case I suppose that your customer order was entered at plant A. Right?
Check in the sales order, at the Procurement tab if the Schedule line category is relevant for the MRP. If it is CN it is not relevant.
If it is relevant for the MRP, then it should work. Delete the planned order that you see at plant A (maybe its is firmed). You should only see a customer order. Then run the MRP again and it should create a purchase requisition.
Try and let me know.
Kind Regards,
Mariano