Some new showcase items were added yesterday, but they are not showing up on the showcase home page. I can still view them and comment them and they show up on he user's profile, but they aren't showing up under the showcase tab.
My guess is that you have IMAGE REQUIRED set for the Home Page options and these new items don't have at least 1 image attachment.
Would be helpful to have screen shots of your HOME PAGE settings (so I can see things like whether or not you have "IMAGE REQUIRED" checked as well as screen shots of the ITEMS to see if they have things like IMAGE ATTACHMENTS uploaded to them.
Also, is there any way to make the images in showcase bigger like how they are in AMS? I changed the thumbnail size in showcase to match how it was in AMS and then rebuilt the showcase thumbnails, but that didn't change anything.
AMS and Showcase are 100% the same when it comes to attachment thumbnails. They both have the same Layouts (Article View, List View, Grid View, Tile View, News View, News View II, Custom View and Custom View II).
Your screen shot above is List View layout (which is a small thumbnail. Switch the Layout Type to Tile View or News View II or Grid View if you want the really large looking layouts.