Ball will stop when it collides with paddle and bounce off once paddle is moved in pong game

I'm coding pong in python for the first time and I'm trying to make the ball bounce when it hits a paddle. Right now, the ball stops when it hits the paddle and then will only bounce off once the paddle is moved. I want the ball to bounce as soon as it hits the paddle. Any idea on how to fix this? Any help is appreciated. Thanks!

import pygame, random, sys
from pygame.locals import *
#set constants
WINDOWWIDTH = 600
WINDOWHEIGHT = 600
TEXTCOLOR = (255, 255, 255)
BACKGROUNDCOLOR = (0, 0, 255)
FPS = 40
BLACK = (0,0,0)
RED = (255, 0, 0)
WHITE = (255, 255, 255)
#set variables
rectY1 = 240
rectY2 = 240
Y1change = 0
Y2change = 0
ballX = 320
ballY = 240
ballXvel = 0
ballYvel = 0
paddle1 = pygame.Rect(18,rectY1,10,120)
paddle2 = pygame.Rect(620,rectY2,10,120)
ball = pygame.Rect(ballX,ballY,30,30)
#create shapes
def drawshapes():   
    pygame.draw.rect(DISPLAY, WHITE, paddle1)
    pygame.draw.rect(DISPLAY, WHITE, paddle2)
    pygame.draw.rect(DISPLAY, WHITE, ball)

#set up display    
pygame.init
DISPLAY = pygame.display.set_mode((640,480),0,32)
pygame.display.set_caption('Pong')
fpsClock = pygame.time.Clock()
#main loop

while True:
    for event in pygame.event.get():
        if event.type==QUIT:
            pygame.quit()
            sys.exit()
        if event.type == KEYDOWN :
            if event.key == K_SPACE :
                ballXvel += 10
                ballYvel += 10
    if ball.colliderect(paddle1) :
        ballX = paddle1.x
        ballXvel = ballXvel * -1

    if ball.colliderect(paddle2) :
        ballXvel = ballXvel * -1




        if ballX > 620 :
            ballXvel = ballXvel * -1
    ##    if ballX < 20 :
    ##        ballXvel = ballXvel * -1 
        if ballY > 460 :
            ballYvel = ballYvel * -1
        if ballY < 20 :
            ballYvel = ballYvel * -1

        keys = pygame.key.get_pressed()

        Y1change = 0
        Y2change = 0
    #make paddles move
        if keys[K_UP]:
            Y2change -= 10

        if keys[K_DOWN]:
            Y2change += 10

        if keys[ord('w')]:
            Y1change -= 10

        if keys[ord('s')]:
            Y1change += 10
    #stop paddles at edges 
        if rectY1 < 5 :
            rectY1 = 5
        if rectY1 > 355 :
            rectY1 = 355
        if rectY2 < 5 :
            rectY2 = 5
        if rectY2 > 355 :
            rectY2 = 355
        ball.y = ballY
        ball.x = ballX
        ballX += ballXvel
        ballY += ballYvel
        paddle1.y = rectY1
        paddle2.y = rectY2
        rectY1 += Y1change
        rectY2 += Y2change
        DISPLAY.fill(BLACK)
        drawshapes()
        pygame.display.update()
        fpsClock.tick(FPS)

Move the ball before you do the collision test not after. Since a step of the ball is great than 1, the ball doesn't hit the paddle exactly. If the ball hits the left paddle, then you have to put the left side of the ball on the right side of the paddle. If the ball hits the right paddle the you have to put the right of the ball on the left of the paddle.
In a similar way you have to restrict the position of the ball to the window:
(Note ballX, ballY is the top left of the ball)

while True:
    # [...]

    ballX += ballXvel
    ballY += ballYvel

    if ball.colliderect(paddle1) :
        ballXvel *= -1
        ballX = paddle1.right

    if ball.colliderect(paddle2) :
        ballXvel *= -1
        ballX = paddle2.left - ball.width

    size = DISPLAY.get_size()
    if not 0 <= ballX < size[0] - ball.width:
        ballXvel *= -1
        ballX = max(0, min(size[0] - ball.width, ballX))
    if not 0 <= ballY < size[1] - ball.height:
        ballYvel *= -1
        ballY = max(0, min(size[1] - ball.height, ballY))

    # [...]

    ball.y = ballY
    ball.x = ballX
    # ballX += ballXvel <-- DELETE
    # ballY += ballYvel