I get sometimes error in my tests

This is the test for this case

defmodule WorkersWeb.Api.V1.CountyControllerTest do
  use WorkersWeb.ConnCase
  alias Workers.Mongo, as: Wmongo
  @token "authtoken123"

  setup do
    Wmongo.delete_many!("counties", %{})

    conn = build_conn()
    |> put_req_header("accept", "application/json")
    |> put_req_header("content-type", "text/json")

    jwt = sign_and_store_user(@token)
    {:ok, conn: conn, jwt: jwt}
  end

  describe "index/2" do
    test "OK", %{conn: conn, jwt: jwt} do
      create_counties()

      conn = conn
      |> put_req_header("authorization", jwt)
      |> get("/api/v1/counties?order_by=name&order_dir=desc")

      json = Poison.decode!(conn.resp_body)

      assert Enum.count(json["counties"]) == 3
      assert Enum.map(json["counties"], &(&1["name"])) == ["Sheridan", "SABINE PASS", "SABINE OTHER"]

      co = List.first(json["counties"])
      assert Map.keys(co) == ["api_id", "name", "state_abbr"]
    end

    test "Unathorized", %{conn: conn} do
      conn = get(conn, "/api/v1/counties")

      assert conn.status == 401
      json = Poison.decode!(conn.resp_body)

      assert json["msg"] == "Please login."
    end
  end

  describe "show/2" do
    test "ok", %{conn: conn, jwt: jwt} do
      api_id = "49-033"
      Wmongo.insert_one!("counties", %{name: "Sheridan", county_api: "033", api_id: api_id, state_abbr: "WY"})

      conn = conn
      |> put_req_header("authorization", jwt)
      |> get("/api/v1/counties/#{api_id}")

      json = Poison.decode!(conn.resp_body)
      assert json["county"] == %{"api_id" => "49-033", "name" => "Sheridan", "state_abbr" => "WY"}
    end

    test "Not found", %{conn: conn, jwt: jwt} do
      conn = conn
      |> put_req_header("authorization", jwt)
      |> get("/api/v1/counties/49-333")
      json = Poison.decode!(conn.resp_body)

      assert json["error"] == "County not found"
      assert conn.status == 404
    end
  end
end

Method to create counties

  def create_counties do
    Mongo.delete_many(:mongo, "counties", %{}, [pool: DBConnection.Poolboy])
    Mongo.insert_many(:mongo, "counties", load_fixture("counties")["counties"], [pool: DBConnection.Poolboy])
  end