Causes:

1. Variations do not have Custom Labels attached to them, therefore Linnworks does not know what they are and there isn't a method of updating inventory for them.
2. Items or Variations do not have stock currently.

Solutions:

1. Add Custom Labels to the variations that do not have them, then go back to Linnworks, Settings → Channel Integration → Mapping → Right Click on the Listings area → Refresh List. This should pull in your variations.
2. For the first instance if the item has not been mapped to linnworks before you will need to relist the item manually for the first time, hen go back to Linnworks, Settings → Channel Integration → Mapping → Right Click on the Listings area → Refresh List. This should pull in your variations. This should only need to be done in the first instance.