What version of Premiere Elements, what computer operating system.
What is this .m4a file - from iTunes?
iTunes music and .m4a files have a long history of importer issues with Premiere Elements independent of the version of Premiere Elements.
The typical answer is to take your .m4a into iTunes to convert it to the WAV version and then import that WAV version into the Premiere Elements project with the project's Add Media (Get Media)/Files and Folders.
Please ask if you need help with the above how to.
Because of the history on this, the why might be why then rather than why now.
Nonetheless, this should work for you and you should be moving forward with your Premiere Elements projects.
Add On...As your schedule permits, please let us know the outcome.