From Joomla! Documentation
Updating to 3.1.0 may lead to the "disappearance" of intro text and images from featured or blog article views. To fix please update to 3.1.1.
If for some reason updating to 3.1.1 is not possible, change "Show Intro Text" to Show. This may temporarily cause your full article to display the intro text when you don't want that. In that case you should change the Show Intro Text for each impacted article to Hide.