There are a few reasons why your perspective search is returning empty results:
1. There are no assets that meet your categorization/search filters.
2. The assets you are categorizing/searching for have already been allocated to a group.
One way to determine the group an asset resides in is by going to the asset report, locating the asset itself and pulling in the 'Group Membership' column.
3. You've reached the group limit for your perspective.
You can find the perspective group limit at the top of your static/dynamic block:
For more information regarding perspective maintenance, see: Manage Perspective Groups