sbt-concat doesn't concatenate files in subfolders

[java...@gmail.com]

Hi all,

i'm trying to create a concat group using sbt-concat in a play framework project (v2.3.8). But it doesn't work. The files in the /app/assets/ folder are concatenated successfully but any group incorporating files in subfolders doesn't work. In the example below group_1.js is created but group_2.js isn't created although after executing the command activator web-stage i see the message Building 2 concat group(s) displayed. What i'm doing wrong? Thank you very much for any hints in advance.

PS: I wanna use the concatenated files in development and production mode. How would i include the files in my template?
Can i just use <script src="@routes.Assets.at("group_1.js")"> ?

Directory Layout:

    + app
    --+ assets
    ------ c.js
    ------ d.js
    -----+ js
    --------+ foo
    ----------- a.js
    ----------- b.js
    ...
    ..
    .

  build.sbt:

        import play.PlayJava

        name := """test"""

        version := "1.0"

        scalaVersion := "2.11.1"

        lazy val root = (project in file(".")).enablePlugins(PlayJava, SbtWeb)

        libraryDependencies ++= Seq(
          javaCore,
          ...
        )

        Concat.groups := Seq(
            "group_1.js" -> group(Seq(
              "js/foo/a.js",
              "js/foo/b.js"
            )),
            "group_2.js" -> group(Seq(
               "c.js",
               "d.js"
             ))
          )

        pipelineStages := Seq(concat)

  plugins.sbt:
  
    logLevel := Level.Warn

    resolvers ++= Seq(
      "Typesafe repository" at "http://repo.typesafe.com/typesafe/releases/",
      Resolver.sonatypeRepo("releases")
    )

    addSbtPlugin("com.typesafe.play" % "sbt-plugin" % "2.3.8")
    addSbtPlugin("com.typesafe.sbt" % "sbt-web" % "1.1.1")
    addSbtPlugin("net.ground5hark.sbt" % "sbt-concat" % "0.1.8")