Hope you are well.
The inventory sync for any channel runs approximately every 10 minutes after the previous one had finished. In order for linnworks to submit a stock level update, a stock change has to occur first. This can be either a manual change in the items stock levels or an incoming order. Basically whenever a stock level in linnworks becomes different to what it is on the channel, linnworks will try and submit the current level it has.
This of course, only works for linked listings and to manually trigger an inventory update you could import a csv file with updated stock levels or opt for the stock level adjust column option in the inventory import - (https://www.linnworks.com/support/settings/import-and-export-data/import-data/import-types/inventory-import).
I have mapped a few items on ebay and shopify and stare at them to see when the quantity will be updated. Nothing happens so far. When will sync take place? Anyway to manually invoke it? Thanks!