Symptoms

The K2 application icon does not appear in the List ribbon from previously “Appified" SharePoint list, even though the app is in the app catalog and the registration wizard runs without any issue.

Diagnoses

This is related to SharePoint Web part being modifying to "Summary Toolbar" type. The default setting is "Full Toolbar".

Resolution

Steps to resolve the issue:
1. Choose edit page from the gear menu.
2. Click edit on list/ library web part (this is the Web Part displaying the actual items on the list or library.)
3. Select "Full Toolbar" for the tool bar type.
4. Click “Apply” (K2 Ribbon Menu items will still show up)