If you Update yout TFS on premise from 2013 to 2015 or from 2015 to 2015 Update 1 you might experience that your bugs are no longer displayed in your backlog.

This is can now be configured for individual teams. Open the settings for your project.

Click on the team that you want to configure.

Got to settings and configure how the bugs are handled in the backlog of the team.

The bugs then appear again on your backlog.

What you shouldn’t do is modify the workitem categories like some posts suggest. If you do this you will lose the capability to configure it per team. If you want this anyways you can export the categories using witadmin, add the bug to the requirements category and import the file again.

witadmin exportcategories /collection:https://<server>/tfs/<collection> /p:<project> /f:C:\temp\categories.xml

 
<CATEGORY refname="Microsoft.RequirementCategory" name="Requirement Category">
    <DEFAULTWORKITEMTYPE name="Product Backlog Item" />
    <WORKITEMTYPE name="Bug" />
</CATEGORY>

witadmin importcategories /collection:https://<server>/tfs/<collection> /p:<project> /f:C:\temp\categories.xml