forAll fails to generate different values

ExampleSpec.scala

import collection.mutable.Stack
import org.scalatest._

import org.scalatest.prop.GeneratorDrivenPropertyChecks
import org.scalacheck.Gen
import org.scalacheck.Gen.{ listOf, Choose }

import org.scalacheck.Gen

// Built & run with:
//  $ scalac -cp scalatest-app_2.11-3.0.1.jar:scalacheck_2.11-1.13.4.jar ExampleSpec.scala
//  $ scala -cp scalatest-app_2.11-3.0.1.jar:scalacheck_2.11-1.13.4.jar org.scalatest.run ExampleSpec

class ExampleSpec extends FunSpec with Matchers with GeneratorDrivenPropertyChecks {
  val nameGen = Gen.alphaNumStr.retryUntil(_.length > 0)

  it("nameGen") {
    forAll(nameGen) { str =>
      str.length should be > (0)
    }
  }

  it("nameGen: sample") {
    val str = nameGen.sample
    str match {
      case None => ??? // just need something here.
      case Some(s) => s.length should be > (0)
    }
  }
}