Visual Studio 2015 new files not being added to source control automatically

Folders called Release and their contents is automatically excluded from TFS (along with Debug and lots of file types). You can override this for particular folders by creating a .tfignore file, as detailed in the “Customize which files are ignored by version control” section of this link

Customize which files are ignored by version control

By default certain types of files (for example, .dll files) are
ignored by version control. As a result:

When you add ignored files to folders that are mapped in a local
workspace, they do not appear in the Pending Changes page in Team
Explorer.

When you try to add ignored files using the Add to Source Control
dialog box (for example by dragging them into Source Control
Explorer), they automatically appear in the Excluded items tab.

You can configure which kinds of files are ignored by placing text
file called .tfignore in the folder where you want rules to apply. The
effects of the .tfignore file are recursive. However, you can create
.tfignore files in sub-folders to override the effects of a .tfignore
file in a parent folder.

.tfignore file rules

The following rules apply to a .tfignore file:

# begins a comment line

The * and ? wildcards are supported.

A filespec is recursive unless prefixed by the \ character.

! negates a filespec (files that match the pattern are not ignored)

.tfignore file example

Ignore .cpp files in the ProjA sub-folder and all its subfolders
ProjA*.cpp

Ignore .txt files in this folder
*.txt

Ignore .xml files in this folder and all its sub-folders
*.xml

Ignore all files in the Temp sub-folder
\Temp

Do not ignore .dll files in this folder nor in any of its sub-folders
!*.dll