Invoice to inventory

[mag8524]

I have created an invoice and an inventory to keep track of my stock. The problem is when i start a new invoice the numbers in the inventory go back to 0. For example: i have ten items in stock a customer buys 2 and 8 are left. When a new customer comes and buys another 1 the total is 9 instead of 7. How to keep the entries from the previous invoice and go on from there?