The default installation of OpenCart has the brands/manufacturers displayed in a carousel at the bottom of the home page. Updating the manufacturers in Admin->Catalog->Manufacturers will not affect that. If you want to change this you’ll need to update the manufacturers’ banner.
To update the banner go to Admin->Setting->Layout->Banner. Here you can update the Manufacturer banner to display the manufacturers you want.
Some people don’t know this and try to play with Carousel module or Manufacturers in Catalog, but they have to update the banners in system layout.