Composer doesn't see that updates are needed My Drupal version is 8.5.1, and yet when I run...

composer outdated drupal/*


It says "nothing to install or update".

What to do?

This is the relevant part of my composer.json...

"require": {
        "composer/installers": "^1.0.21",
        "wikimedia/composer-merge-plugin": "~1.3",
        "drupal/core": "^8.5"
    },


Looks fine to me, but composer says no update is available.
Update:  This is not a good answer.  Leaving it here for reference, but this does not solve the underlying problem, whatever it is.

Fixed by deleting vendor and core directories and composer.lock and then running...

composer update drupal/core --with-dependencies

It seems there must be some circumstances when the 'composer outdated' command doesn't work, but it's still possible to upgrade.