为什么导管不动？

为什么管道在接触窗口的开头时不会移动到末端？

import pygame

pygame.init()

sc = pygame.display.set_mode((700,800))
pygame.display.set_caption("Flappy Bird")
FPS = 30
clock = pygame.time.Clock()

x = 350
y = 400

xOfDuct = 732
yOfDuct = 800 

flag = 0

def duct():
    ductOne_surf = pygame.image.load('trubaDown.png')
    ductOne_surf = pygame.transform.scale(ductOne_surf, (120,452))   
    ductOne_rect = ductOne_surf.get_rect(bottomright=(xOfDuct,yOfDuct))
    sc.blit(ductOne_surf,ductOne_rect)
def ductTwo():
    duct_surf = pygame.image.load('trubaUp.png')
    duct_surf = pygame.transform.scale(duct_surf, (120,452))
    duct_rect = duct_surf.get_rect(bottomright=(xOfDuct, yOfDuct - yOfDuct + 175))
    sc.blit(duct_surf,duct_rect)

while True:
    sc.fill((255,255,255))
    duct()
    ductTwo()
    pygame.draw.circle(sc, (255,255,0),(x,y),20)

    for i in pygame.event.get():
        if i.type == pygame.QUIT:
            exit()
    
    keys = pygame.key.get_pressed()     

    if keys[pygame.K_SPACE]: 
        flag = 0
        y-=15
    if xOfDuct == 0:
        xOfDuct = 700       
    flag+=2
    y+=flag     
    xOfDuct -= 10

    pygame.display.update()             
    clock.tick(FPS)