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- Double check that Flat Product Catalog and Flat Category Catalog is enabled in System → Configuration → Catalog → Frontend. More information on this topis you may find here.
- Navigate to the attribute's detail in Catalog → Attributes → Manage attributes. Then, set the option Use in product listing to Yes and save attribute details.
- Finally, reindex Product Flat Index in System → Index management and refresh cache. Sometimes you have to do this step twice.
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