WHY IS THERE SO MANY NEGATIVE WHERE MY SYSTEM SETTING IS WHEREBY IF THERE IS NO STOCK I COULDNT ISSUE INVOICE

WHY IS THERE SO MANY NEGATIVE WHERE MY SYSTEM SETTING IS WHEREBY IF THERE IS NO STOCK I COULDNT ISSUE INVOICE

Pls note that checking of Negative Stock is always "as on today" and it does not check retroactively. 

For example:
--
10 Jul - QOH = 10 pcs
19 Jul - QOH = 20 pcs
--
If on 21 Jul, user enters an invoice for 15 pcs, system will allow. If the user enters the date as 15 Jul, it will still allow because the check for QOH is as on 21 Jul. In this instance, the system will record a Negative Stock on 15 Jul of  -5 pcs.