The Products and the Assets visibility on the portal are managed by two different channels.
In the Portal, when you are on the product 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 included, somewhere, in one of the selections (manual or dynamic) for this channel on your PIM:
The type of link between the product and the asset must be ticked in the product channel of the portal on your PIM:
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 Product Channel assigned to your Portal.
Finally, be sure to select the correct link type for the asset thumbnail :
Keywords: asset, portal, channel.