We are providing a custom fulfillment service to some shops. We manage the inventory in a separate backend, and provide the inventory levels to the store on request.
Shopify polls the fetch_stock.json endpoint every hour and we send a response listing each SKU and the inventory level (taking into account unfulfilled orders).
However, Shopify does not appear to always update the inventory level for all the SKUs.
In one case, a new product was added and correctly had it's level set to 2, a while later the stock was increased by 2 and this was reflected by the fetch_stock response showing 4 against that SKU, but the inventory level in Shopify did not increase until a manual update was made to the product.
This has happened mostly in the stores which have several hundred products, and does not seem to happen when a new product is added. Is it possible that there are too many items in our responses? Or is it something else?