How can I efficiently update inventory levels on my drop shipping app?

[ChrisWHG]

I work for a wholesale company and a few of our customers are using Shopify, so I am developing an App to handle the drop shipping. I have never worked with Shopify before so I have set up a Test Store.

So far I have registered a Fulfillment Service. When I retrieve it from the API it looks like this:

{"fulfillment_services":[{"id":62906630451,"name":"WHG Drop Shipping","email":null,"service_name":"WHG Drop Shipping","handle":"whg-drop-shipping","fulfillment_orders_opt_in":true,"include_pending_stock":false,"provider_id":null,"location_id":75071193395,"callback_url":"https:\/\/www.whg.net.au\/dropship\/shopify\/orders\/","tracking_support":true,"inventory_management":true,"admin_graphql_api_id":"gid:\/\/shopify\/ApiFulfillmentService\/62906630451"}]}

I have set up product importing and have a number of products with the "fulfillment_service" and "inventory_management" set to "whg-drop-shipping".

My problem is trying to update the inventory levels. I have set up the fetch_stock callback, it can be seen here https://www.whg.net.au/dropship/shopify/orders/fetch_stock.json?location_id=123&shop=whg-test-store..... The return at the moment is:

{"WHGBGPR5":22,"WHGBGPR1":422}

But it has never been called. I read it is supposed to update hourly?

I have also tried setting inventory levels through the API.

{"location_id":"75071193395","inventory_item_id":"46172169699635","available":"422","relocate_if_necessary":true,"disconnect_if_necessary":true}}

But the response is:

{"errors":{"inventory_item_id":"Required parameter missing or invalid"}}

The inventory_item_id is correct, the location_id is correct. Any ideas why it is sending this error?