Why is TFS ignoring a project in my solution?

Your project doesn't have appropriate Source Control Bindings! With the solution open, you'll need to go to File -> Source Control -> Change Source Control to view the bindings for your projects. The failing one probably will show an "Unknown" or not binded status. You can then use that dialog to correct the binding and check in the project file.

Appended Alternate Solution - Navigate to the Team Explorer -> Source Control and manually add the files using the "Add Items to Folder" menu item.

You can try following steps

1 Unbind the project in "Change Source Control" dialog

2 Refresh project to update source control status

3 Right click the project and click Add the Project to source Control in solution explorer