Civi 5.9 not serving extension javascript in WP 5
WordPress/CiviCRM can have trouble resolving tokens like [civicrm.files] if you have a symlink in your path.

If you're having resource URL issues, I recommend replacing the tokens with absolute paths (with no symlinks) in Administer » System Settings » Directories as well as checking Administer » System Settings » Resource URLs as Aidan suggests.  It sounds like your CIVICRM_UF_BASEURL is set correctly in civicrm.settings.php but it never hurts to check.

A permanent solution for this is underway: See https://lab.civicrm.org/dev/core/issues/1497.