Eclipse not updating referenced libraries for my Play framework application
You have to redo an eclipsify in the Play console after adding your new dependency (ie after updating your Build.scala file).
It will generate a new .classpath file for Eclipse containing your new dependency.
If there is Build.scala set up properly, and an info about required library found in newly-generated .classpath file, than try to run play eclipsify and if it works NOT, simply restart Eclipse. It worked for me.