Fulfillment Service fetch_stock works for one sku but not for many

[csan]

Hello,

we are trying to use the fetch_stock option as part of the fulfilment services.
fetch_stock works well when updating specific sku, but when shopify is submiting the hourly calling fetch_stock w/o the sku,  not all skus are updated. moreover, Shopify will call our script multiple  times, basically every one second fro about 10-15 minutes.
The process to generate the json is fast (1 sec or less)

we tried feeding also header 'Content-Type: application/json', but it looks like Shopify is unable to get the full JSON and therefore is trying again and agian - the JSON is not very big - from few skus to 3K)

How long does it take for Shopify to process the JSON feed ? any known time or size limit for the fetch_stock process.
ANy idea how to identify why Shopify is unable to get the data ?

any help or directions will be greatly appreciated.
 

Thanks
Coby