Hi Joshua,
You may also find some information in my blog related to securing warehouses (and related tables). kaya-consulting.com/extensible-data-security-examples-secure-by-warehouse
You can find an example for download, so you can also compare your efforts with it.
If the policy is not working, it might be caused to the query or possibly another overlapping policy. Also check if the lookup form is actually presenting data using the InventSite as datasource or if another table name from e.g. a view is used.