Web scraping from .aspx site using python

I'm attempting to scrape some data from this site: https://fortress.wa.gov/esd/file/warn/Public/SearchWARN.aspx

I am able to get the first 11 pages using my method but for some reason it quits beyond the 11th page. I've read the other posts related to .aspx and haven't seen anything that applies to my situation.

I'm new to this so my code is a little verbose but it gets the job done--somewhat. I've played with adjusting the headers and a bunch of other stuff but cannot get past the 11th page. Makes no sense to me.

I'm fairly certain the problem lies in the viewstate and viewgenerator header parameters. I'm not sure how to get these for the page you want to go to in the loop. I'm pretty much using the same values for all pages. For some reason, this approach works up to and including page 11 then it breaks. This is odd since it looks like each page has a different viewstate value.

Thanks in advance.

import pandas as pd
import re
import pandas as pd
import numpy as np
import urllib
from requests import Session
from bs4 import BeautifulSoup
import time
import requests


# List of pages to loop over
page_list = ['Page$1','Page$2','Page$3','Page$4','Page$5','Page$6','Page$7','Page$8','Page$9','Page$10',
             'Page$11','Page$12','Page$13','Page$14','Page$15','Page$16','Page$17','Page$18','Page$19','Page$20']
wa_url = 'https://fortress.wa.gov/esd/file/warn/Public/SearchWARN.aspx'

# Getting header elements from url
session = requests.Session()
session.headers.update({
    "user-agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.97 Safari/537.36"
})
val_get = session.get(wa_url)
soup = BeautifulSoup(val_get.content, "html.parser")

tags = soup.find_all('input')
# Header elements I need for the POST request
view_state = tags[3]['value']
view_generator = tags[4]['value']
evnt_validation = tags[6]['value']



no_emps = []
date = []

#Looping through pages of WARN database
for page in page_list:
    
    data = {
    # Form data header stuff
    "__EVENTTARGET": "ucPSW$gvMain",
    "__EVENTARGUMENT": page,
    "__LASTFOCUS": "",
    "__VIEWSTATE": view_state,
    "__VIEWSTATEGENERATOR": view_generator,
    "__VIEWSTATEENCRYPTED": "",
    "__EVENTVALIDATION": evnt_validation,
    "Accept":"text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9",
    "Accept-Encoding":"gzip, deflate, br",
    "Accept-Language":"en-US,en;q=0.9",
    "Cache-Control":"max-age=0",
    "Connection":"keep-alive",
    "Content-Type":"application/x-www-form-urlencoded",
    "Cookie":"_ga=GA1.2.1011893740.1592948578; _gid=GA1.2.1433455844.1592948578",
    "Host":"fortress.wa.gov",
    "Origin":"https://fortress.wa.gov",
    "Referer":"https://fortress.wa.gov/esd/file/warn/Public/SearchWARN.aspx",
    "Sec-Fetch-Dest":"document",
    "Sec-Fetch-Mode":"navigate",
    "Sec-Fetch-Site":"same-origin",
    "Sec-Fetch-User":"?1",
    "Upgrade-Insecure-Requests":"1"
    }
    
    # Getting data from each page
    session = requests.Session()
    session.headers.update({
        "user-agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.97 Safari/537.36"
    })
    
    get_warn_data = session.post(wa_url, data=data)
    soup = BeautifulSoup(get_warn_data.content, "html.parser")
    
    # Getting all rows of data and desired table data after some cleaning up
    work = soup.find_all('tr')
    work = [a.get_text('@') for a in work]
    work = [re.sub(r'\n', '', a) for a in work]
    work = [re.sub(r'^@|@$', '', a) for a in work]
    work = [a.split('@') for a in work]
    
        
    work = [a for a in work if len(a) == 7]
    no_emps_u = [a[3] for a in work]
    date_use = [a[6] for a in work]
    
    no_emps.append(no_emps_u)
    date.append(date_use)
    
# Dynamically Updating header values with stuff in current html
# Only applicable for page2 and on
if page != 'Page$1':
    tags = soup.find_all('input')
    view_state = tags[3]['value']
    view_generator = tags[4]['value']
    evnt_validation = tags[6]['value']
else:
    pass
    
# Wrapping up results into lists
from pandas.core.common import flatten
WA_WARN_no_emps = list(flatten(no_emps))
WA_WARN_date = list(flatten(date))

Update<<<: Following Andrej's advice in the comments, I added the if statement at the end of the for loop that updates the header values; this addition fixed the code and gets all the pages in page_list.