DNS record for help.example.com returns 404

I have a domain example.com. I'd like to map help.example.com to example.com. The DNS records are:

Hostname           Type     Value
example.com        A        93.184.216.34
help.example.com   A        93.184.216.34
www.example.com    CNAME    example.com

I can access the website with both example.com and www.example.com but when I try help.example.com I get 404 Not Found.:

Not Found
The requested URL / was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

So I'd like to know why is the A record for help.example.com not redirecting to example.com.

Extra information:

My .htaccess consists of the single line:
```
ErrorDocument 404 "Cannot find that URL"
```
If I try to access example.com/some_random_string I get (as expected) Cannot find that URL. However if I try to access help.example.com/some_random_string I get the above Not Found message. So it seems that .htaccess is ignored by help.example.com
Pinging example.com, www.example.com, and help.example.com returns the same IP address.
I also tried to replace the A record with:
```
help.example.com   CNAME     example.com
```
But this returns the same Not Found message.

Solution

Your problem is in webserver configuration. Not in DNS at all.
I assume you use Apache so you need to tell Apache that example.com www.example.com help.example.com have same root directory.
For example if you use VirualHost configuration and Ubuntu then look in /etc/apache2/sites-enabled/example.com.conf
It should have

<VirtualHost *:80>
    ServerAdmin admin@example.com
    ServerName example.com
    ServerAlias www.example.com help.example.com
    ...
</VirtualHost>

If you use other OS/configuration/WebServer you can google/look how your configuration works and where to add your domain name. Not in DNS.