Do you have any manufacturer with more than 1 Product?
By default, the Manufacturer sidebox will only show those with Products ... Configuration ... Maximum Values ...
Manufacturers List - Verify Product Exist
Verify that at least 1 product exists and is active for the manufacturer name to show
Note: When this feature is ON it can produce slower results on sites with a large number of products and/or manufacturers
0= off 1= on
Then, if there is only 1 Product for the Manufacturer and you have the Skip Product Listing on Categories with 1 Product you will go to the Product _info page ...
Configuration ... Layout Settings ...
Skip 1-prod Categories
Skip single-product categories
If this option is set to True, then if the customer clicks on a link to a category which only contains a single item, then Zen Cart will take them directly to that product-page, rather than present them with another link to click in order to see the product.
Default: True