Inventory count error

I have just noticed that the inventory quantity of an item remains static if entering/adjusting transactions for an earlier date, and the quantity is changed to earlier dates

eg i should have 10 in stock, and a month ago i had 15 and i sold 5. 

I had to adjust an invoice to increase the quantity sold by 1 to 6 and now the quantity i have is still 10, but is says a month ago i had 16. 

This is going to have major issues to my balance sheet. Is there anyone who knows why this is happening ? 

thank you

Sylvia