The Documents and the Assets visibility on the portal are managed by two different channels.
In the Portal, when you are on the document page, The linked Assets are displayed here.
But in order for a linked Asset to be displayed in this section :
An Asset Channel must be configured in the Portal administration:
And this Asset must be present, somewhere in the classification of this Asset's Channel :
The link type of the document - asset link must be checked in the Portal Document channel :
So it is important to check that your Asset is present in the Asset Channel classification assigned to your Portal and that the Document - Asset link type is checked in the Document Channel assigned to our Portal.
Finally, be sur to select the correct link type for the asset thumbnail :
Keywords: asset, portal, channel.