Make sure attachments are set to be shown in the related categories' settings.

Alternatively, your Joomla template may override K2 and even worse, it may be missing the code for attachments in the "item" view.

Learn more about K2 templating here getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates and then compare the source files in K2 with any files your template may include already.