Why isn't the ball bouncing back?

I'm making a simple pong game, but when the ball drops from the ceiling, it doesn't bounce back and forth. It just goes off the screen. I can't figure out why this is happening! I am only concerned with the ball bouncing off the top and bottom of the screen and I want it to bounce back and forth in a straight path. Any help is appreciated!

EDIT: I've found the problem! Thanks for all the help!

Here's my base code:

import math
import random
import sys, pygame
from pygame.locals import *

import ball
import colors
import paddle

# draw the scene
def draw(screen, ball1, paddle1) :
   screen.fill((128, 128, 128))
   ball1.draw_ball(screen)
   paddle1.draw_paddle(screen)

#function to start up the main drawing
def main():

   pygame.init()
   width = 600
   height = 600
   screen = pygame.display.set_mode((width, height))

   ball1 = ball.Ball(300, 1, 40, colors.YELLOW, 0, 5)
   paddle1 = paddle.Paddle(250, 575, colors.GREEN, 100, 20)

   while 1:
      for event in pygame.event.get():
         if event.type == QUIT: sys.exit()
         elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_RIGHT:
               paddle1.update_paddle('right', 20)
            if event.key == pygame.K_LEFT:
               paddle1.update_paddle('left', 20)

      ball1.test_collide_top_ball(600)
      ball1.test_collide_bottom_ball(0)
      ball1.update_ball()
      draw(screen, ball1, paddle1)
      pygame.display.flip()

if __name__ == '__main__':
   main()

And here is my code for the ball class/methods:

import pygame

class Ball:
   def __init__(self, x, y, radius, color, dx, dy):
      self.x = x
      self.y = y
      self.radius = radius
      self.color = color
      self.dx = dx
      self.dy = dy

   def draw_ball(self, screen):
      pygame.draw.ellipse(screen, self.color,
         pygame.Rect(self.x, self.y, self.radius, self.radius))

   def update_ball(self):
      self.x += self.dx
      self.y += self.dy

   def test_collide_top_ball(self, top_height):
      if (self.y >= top_height):
         self.dy *= -1

   def test_collide_bottom_ball(self, coll_height):
      if (self.y >= coll_height):
         self.dy *= -1

Your test collide functions return the value of velocity. You never use it anywhere.

You call the update ball with dx=0 dy=5.

Instead of returning the a value after collision, it would be better to hold the dx and dy in the object. So it would become:

class Ball:
   def __init__(self, x, y, radius, color):
      self.x = x
      self.y = y
      self.radius = radius
      self.color = color
      self.dx = 0
      self.dy = 5

   def draw_ball(self, screen):
      pygame.draw.ellipse(screen, self.color,
         pygame.Rect(self.x, self.y, self.radius, self.radius))

   def update_ball(self):
      self.x += self.dx
      self.y += self.dy

   def test_collide_top_ball(self, top_height):
      if (self.y >= top_height):
         self.dy *= -1

   def test_collide_bottom_ball(self, coll_height):
      if (self.y >= coll_height):
         self.dy *= -1