Product inventory not updating

[JoannaS]

Hey, 

im rookie but last 3y I was quite happy with the site I have built. Since I moved up from basic package I’m meeting more and more curious cases where products are not updating inventories. 
My drops are quite small and selling fast: within minutes after release, so inventory updates on time are critical. 
so! Eg: I’m adding product quantities, switching on „track selling” and ensuring „continue selling when out of stock” is off. 
but after I add products# on the product chart and save it I still see 0 on the page and collection page, while on the product specific page # is let’s say 10 (updated). 
the same when customers buy system is not updating stocks and it allows selling below stocks, despite the option switched on. 

don’t know what to do. 
support is useless. 

help!

[Nilankar]

As per less info
Inside Inventory 
Add 0 to selected product, save and re-enter the on hand and Available to same value