Why are some pages not crawling?

I am crawling reviews from the Amazon site.

Something I felt crawling reviews.

Sometimes, you can not crawl your pages. What's wrong?

1. Do I have to hold a long time sleep?

2. Do you not crawl because of the photos in the review? I am sorry, but I exclude certain parts and crawl. I do not know this phrase. If you do not mind, can you teach me?

3. Is there another reason?

   import re, requests, csv
   from bs4 import BeautifulSoup
   from time import sleep
   
   ##CMD : chcp 65001
   
   
   def reviews_info(div):
       review_text = div.find("div", "a-row review-data").span.text 
       review_author = div.find("a", "a-size-base a-link-normal author").text
       review_stars = div.find("div", "a-row").a.text
       on_review_date = div.find('span', 'a-size-base a-color-secondary review-date').text
       review_date = [x.strip() for x in re.sub("on ", "", on_review_date).split(",")]
       return {
           "review_text" : review_text,
           "review_author" : review_author,
           "review_stars" : review_stars,
           "review_date": review_date
       }
   
   base_url = "https://www.amazon.com/GRACE-KARIN-BoatNeck-Sleeveless-Vintage/product-reviews/B016XUCYZO/ref=cm_cr_dp_d_show_all_top?ie=UTF8&showViewpoints=1&sortBy=helpful&pageNumber="
   
   reviews = []
   NUM_PAGES = 472
   
   for page_num in range(1, NUM_PAGES + 1):
       print("souping page", page_num, ",", len(reviews), " data")
       url = base_url + str(page_num)
       soup = BeautifulSoup(requests.get(url).text, 'lxml') 
   
       for div in soup('div', 'a-section review'):
       reviews.append(reviews_info(div))
       sleep(30)
       ###################################################
       #  Save dict data
   keys = reviews[0].keys()
   with open('amazon_GRACE KARIN BoatNeck Sleeveless Vintage Tea Dress with Belt_review.csv', 'w', encoding="utf-8") as f:
       dict_writer = csv.DictWriter(f, delimiter=',', lineterminator='\n', fieldnames=keys)
       dict_writer.writeheader()
       dict_writer.writerows(reviews)