customer order problem

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Hi,

 

I noticed a problem in my site powered by prestashop. when a customer orders a product and confirms his order, I noticed that the stock of the product I have put does not decrease unless I too confirm his order.. this will cause a problem.. suppose the stock is having only 5 items and in the same hour 10 customer ordered the same item and confirmed their orders, in this situation I can confirm the order for 5 of them only while the I'll have to send an apologize message to the rest. so is there a way to solve this problem?