Why is my Scalacheck test with a custom Generator failing after discarding many cases, and how do I fix this?

I am a newbie with Scala and I am writing my first Scalacheck suite.

I have a data structure in my program that essentially looks like a (List[Double], List[Double]) which is well-formed only if each element of _1 is strictly greater than the corresponding element of _2.

Since it's slightly more complicated in practice (although for the purpose of this MWE we can pretend there's all there is to it), I've written a custom generator for it.

I then added two trivial tests (including the most trivial of all, 1 == 1) and in both cases the tests fail, with the messageGave up after only XX passed tests. YYY tests were discarded.

Why is it, and how do I fix it?

Attached is my test suite and the output.

---

package com.foo.bar

import org.scalacheck._
import Prop._
import Arbitrary._

object FooSpecification extends Properties("FooIntervals") {

  type FooIntervals = (List[Double], List[Double])

  /* This is supposed to be a tuple of lists s.t. each element of _1
   *  is < the corresponding element of _2
   * 
   *  e.g. (List(1,3,5), List(2,4,6))
   */

  implicit def arbInterval : Arbitrary[FooIntervals] =
    Arbitrary {
      /**
        * Yields a pair (low, high) s.t. low < high
        */
      def GenPair : Gen[(Double, Double)] = for {
        low <- arbitrary[Double]
        high <- arbitrary[Double].suchThat(_ > low)
      } yield (low, high)

      /**
        * Yields (List(x_1,...,x_n), List(y_1,...,y_n))
        * where x_i < y_i forall i and 1 <= n < 20
        */
      for {
        n <- Gen.choose(1,20)
        pairs : List[(Double, Double)] <- Gen.containerOfN[List, (Double, Double)](n, GenPair)
      } yield ((pairs.unzip._1, pairs.unzip._2))
    }

  property("1 == 1") = forAll {
    (b1: FooIntervals)
    =>
    1 == 1
  }

  property("_1.head < _2.head") = forAll {
    (b1: FooIntervals)
    =>
    b1._1.head < b1._2.head
  }
}

---

[info] ! FooIntervals.1 == 1: Gave up after only 32 passed tests. 501 tests were discarded.
[info] ! FooIntervals._1.head < _2.head: Gave up after only 28 passed tests. 501 tests were discarded.
[info] ScalaTest
[info] Run completed in 1 second, 519 milliseconds.
[info] Total number of tests run: 0
[info] Suites: completed 0, aborted 0
[info] Tests: succeeded 0, failed 0, canceled 0, ignored 0, pending 0
[info] No tests were executed.
[error] Failed: Total 1, Failed 1, Errors 0, Passed 0
[error] Failed tests:
[error]     com.foo.bar.FooSpecification