Hello brother's

Please brother it's very important for me find solution for this issue 

I use odoo v8

When i create  pos order and print invoice from pos screen and make new order for new customer no change in stock quantity

Ex:

Customer: a,

Product: br2, quantity in warehouse: 20

Customer buy 4 qty from br2 product

Now I make order for customer and print in voice for him

When I back to check br2 product quantity I found it still 20 and no change happened

So How I Fix it ? ( Let affect to inventory when make pos order / invoice)

I try both product type (Stockable & consumable)

Regards