why didn't my program write some of the fields?

Question

So I wrote a program to write the different attributes like title, total number of comments, total upvotes, post body, author name of submissions in a CSV file. Code:

class ptdata(Thread):

def run(self):
  print('opening post_data csv')
  
  
  with open('post_data.csv', 'a') as f:
        headers = [
            'ID', 'Date_utc', 'Upvotes', 'Number of Comments', 'Subthread name', 'Post Author'
        ]
        writer = csv.DictWriter(f,
                                fieldnames=headers,
                                extrasaction='ignore',
                                dialect='excel')
        writer.writeheader()
        for post in subreddit.stream.submissions():
            #print(post.title)
            
            data = {
                    "ID": post.id,
                    "Date_utc": post.created_utc,
                    "Upvotes": post.ups,
                    "Number of comments": post.num_comments,
                    "Subthread name": post.title,
                    "Post Auther": post.author
                    }
            
            writer.writerow(data)
            print('writing post row')
            # print(data)

But when I looked into the data it wrote:

       ID      Date_utc Upvotes Number of Comments                                     Subthread name Post Author
0     sw73ml  1645266563.0       2                NaN  I fucking love cars, but actually driving, or ...         NaN
1     sw73sa  1645266581.0       3                NaN                               It's my birthday!!!!         NaN
2     sw73va  1645266588.0       3                NaN                            My bike just got stolen         NaN
3     sw73x0  1645266593.0       4                NaN                   I feel like an outsider socially         NaN
4     sw75gk  1645266754.0      10                NaN     Hallo? Ist dis Bert und Ernie’s BDSM emporium?         NaN
...      ...           ...     ...                ...                                                ...         ...
7703  uou8wd  1652455643.0       2                NaN                       Holy crap I forgot how good…         NaN
7704  uou8yy  1652455648.0       4                NaN                       Just got told to kill myself         NaN
7705  uou8zv  1652455650.0       4                NaN                                            hey YOU         NaN
7706  uouagi  1652455771.0       1                NaN                           STEVEN UNIVERSE IS GREAT         NaN
7707  uouaks  1652455780.0       1                NaN  drinking water after chewing gum is the cold e...         NaN

it is something like this. As you can see it shows NaN in the number of comments and post author column. But When I looked at a different CSV file for which I used the same code, it had written the Author name but not the number of comments.

why is this happening?

Also is there any way to get all that data back? because this bot is running for more than 3 months now and without this information this data is kinda useless