Composer won't update Drupal Core, can't determine what is stopping it I am trying to update Drupal Core 8.6.3 to 8.6.5 (which is the latest Core update at time of writing this).

Using this CLI Composer command:

composer update drupal/core --with-dependencies


completes the whole process without errors.

But after clearing Drupal's cache and checking Status Report, it still shows the previous version (8.6.3).

I'm assuming a dependency may be blocking the Core update.

To test this I ran this CLI Composer command:

composer prohibits


and received this error:

[Symfony\Component\Console\Exception\RuntimeException]  
  Not enough arguments (missing: "package").


After searching a few forums I could not find a definite explanation of that error's meaning or how to address it.

UPDATE - As per my answer below, I found a solution which was:

I noticed in a few threads that this inclusion kept being referred to as a potential Core update issue:

webflo/drupal-core-require-dev


So I checked my composer.json require list and it was there, specifying the earlier Core version:

    "require-dev": {
    "webflo/drupal-core-require-dev": "8.6.3"
},


To correct this, I ran this CLI Composer Command:

composer update drupal/core webflo/drupal-core-require-dev --with-dependencies


After clearing cache (drush cr) I checked back on my composer.json and the require had changed to:

    "require-dev": {
    "webflo/drupal-core-require-dev": "^8.6.0"
},


This then triggered a Status Report Warning that 2 Tables required update.
To fix this I ran this CLI Command:

drush updatedb


I then successfully run this CLI Composer command:

composer update drupal/core --with-dependencies


After another cache clear, I checked Drupal's Status Report which showed the update to Core had worked.
I was happy with this so (as I was in maintanenace mode during this time) I then used this CLI command to turn off maintenance mode: 

drush sset system.maintenance_mode 0


And all is well and working correctly.
I found some other threads that highlighted webflo/drupal-core-require-dev as being an issue,
In CLI I ran

composer show


This showed me that webflo/drupal-core-require-dev was set to 8.6.3

So I ran update in CLI:

composer update drupal/core webflo/drupal-core-require-dev --with-dependencies


This updated the require version to 8.6.5.
Upon checking Drupal Status Report it also triggered a warning requiring Database updates, so I ran:

drush updatedb


and approved"yes" to the 2 updates.

This removed the Database error from Status Report and I then ran the core update CLI again:

composer update drupal/core --with-dependencies


This time it worked, so I cleared cache and turned off maintenance mode:

drush cr
drush sset system.maintenance_mode 0


and the Status Report is now clear of errors and warnings.