There are two main reasons as to why the Asset you are searching for may not be showing up.
Archived Asset
The Asset you are searching may have previously been archived.
To view and/or unarchive an Asset, see the below article for instructions.
Incorrect Project Allocation
A User may be unable to view an Asset if they only have access to particular Projects. In this case they will only be able to view the Assets linked to those Projects.
To update this, follow the below article.
If an Asset has been linked to the incorrect Project or is linked to more than one Project, this can be edited by a User with permission to do so.
Please see the below article for further instruction.