Why does my product quantity keep resetting in my online store?

[Bill98]

I have a little issue, every time I changed the quantity of my stock in Shopify it works for few days and then says out of stock and I have to every time change the quantity of my product 

Remind: my first supplier on Ali express is out of stock so I change the supplier to a new one in dsers and I mapped the products but I don’t know why is still doesn’t work in my Shopify store