IF command in python is only running once and not repeating. How can I fix this so it repeats?

I'm very new to python and Pygame and am trying to create a game. I would like a "powerup" to appear after a set time, then repeat every time the timer hits so many seconds.

In the code, the "powerup" appears and drops down the screen after 5 seconds, the timer resets, but the next time the timer reaches 5 seconds, it continues to count and no "powerup" is produced.

I've spent hours trying to work out why this isn't working.

Any help very gladly received, thank you.

import pygame
import random

WIDTH = 480
HEIGHT = 600
FPS = 60

WHITE = (255, 255, 255)
BLACK = (0, 0, 0)
GREEN = (0, 255, 0)

pygame.init()
screen = pygame.display.set_mode((WIDTH, HEIGHT))
clock = pygame.time.Clock()

time_difference = 0

font_name = pygame.font.match_font('arial')
def draw_text(surf, text, size, x, y):
    font = pygame.font.Font(font_name, size)
    text_surface = font.render(text, True, WHITE)
    text_rect = text_surface.get_rect()
    text_rect.midtop = (x, y)
    surf.blit(text_surface, text_rect)

class HealthPack(pygame.sprite.Sprite):
    def __init__(self):
        pygame.sprite.Sprite.__init__(self)
        self.image = pygame.Surface((50, 40))
        self.image.fill(GREEN)
        self.rect = self.image.get_rect()
        self.radius = 50
        self.rect.x = random.randrange(WIDTH - self.rect.width)
        self.rect.y = random.randrange(-150, -100)
        self.speedy = 6
        self.speedx = 0

    def update(self):
        self.rect.y += self.speedy
        if self.rect.top > HEIGHT:
            self.kill()

all_sprites = pygame.sprite.Group()

healthpack = HealthPack()

running = True
while running:
    clock.tick(FPS)
    time_difference = pygame.time.get_ticks()
    if time_difference >= 5000:
        all_sprites.add(healthpack)
        time_difference -= 5000
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    all_sprites.update()

    screen.fill(BLACK)
    all_sprites.draw(screen)
    draw_text(screen, str(time_difference), 18, WIDTH / 2, 55)

    pygame.display.flip()

pygame.quit()