I tried to figure this one out a long time ago but I had no luck back then. Today I've found a way. When you publish more than a post for a given day, only the most recent post (last post) will have it's date shown. Moreover, certain templates (like Awesome Inc) make it worse by grouping all these posts together as though they are all one single post. Although it makes sense to group posts published on the same day under a single category, lot's of users prefer to have the date shown on each post separately.