After making changes to asset attributes, you'll often need to reindex your assets for the changes to take effect. To do this, log into to your Asset bank as an admin users and navigate to:
Admin > Attributes > Manage Attributes
The link to run a reindex is in the top paragraph and called reindex existing items.
There are two options, online and offline.
With an online reindex, the index for each asset is rebuilt individually. It makes the reindex process slower overall, but also means that the majority of your assets will be available while the process runs.
An offline reindex removes the index for all assets, and then as each asset is reindexed it will reappear in your Asset Bank. Although the process is faster, with an offline reindex your assets won't be available until they've been reindexed, therefore this type of reindex is more disruptive.