When a website redirects the visitor to a new location at the initial request, often the redirect strips any URL parameters, such as the affiliate code.
To see if your website is doing a redirect and stripping parameters, this command can be run from an OSX Terminal session to check if the website has a direct:
curl -si "http://www.leaddyno.com?afmc=123" | grep -i "location:"
If there is a redirect, you will see a result like:
Which, in this case indicates a redirect from http to https, but the query parameters are preserved across the redirect, so this situation is ok. If the "Location:" value returned did not have the same ?afmc=123 on the end, that means the redirect would be removing the affiliate information, and the referring affiliate would be lost.
Configuring the web server to redirect and preserve parameters is outside the scope of this article, as there are many different server software packages and ways to implement this change. If your website has this issue, bring this up with your website administrator and let them know the site must preserve the URL parameters across the redirect.