TFS is blind of changes if you have not synched upto the latest version.
Example you worked on page A. You went on vacation. A college worked on page A when you were out. You came back. You know that your college updated Page A. You select the Page A and compare against the latest version. It does not show any modification. This should because the TFS has a latest version which has update contents.
If you select the history of the file, it will correctly show the latest versions, including the modification by your collegue. But the TFS is simply blind of any latest version that you have not synched upto. It does how the red check mark (for modified files), it just shows the contents are the same which aren’t.