My guess is that you did not "break" your content into introtext and fulltext. You do that by clicking the "read more" button in the WYSIWYG editor in K2, which is common in Joomla articles and other components.

As a workaround, you can enable word limiting in the K2 category parameters.

Great, although in the long-term it's best to always have your content split to into/fulltext.