Although this might seem like a complex or very general problem with many solutions, in most cases it's easily solved by eliminating possible causes. First of all make absolutely sure that your WML code is correct. Use a very simple WML deck to do this, something like below that every browser will render without any problem.

<?xml version="1.0"?>
<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN" "http://...">
<wml>
 <card id="test">
  <p>Hello world</p>
 </card>
</wml>

If that doesn't work, check that the web server you are using is configured for the correct MIME types. Because the deck works for you, this is most likely not the problem, but some emulators will actually render decks even though the emulator received an incorrect MIME type. To verify that the MIME type is correctly configured, simply use Netscape or Internet Explorer and point to the URL where the deck is. Netscape and Internet Explorer will of course not be able to display the deck correctly, but both will tell you that they have received data of type text/vnd.wap.wml. If they do, the MIME types are configured correctly.

The next step is to verify that people on the internet can actually reach your web server. Figure out where your web server is located. If you're at work, your employer might have a firewall in place that prevents people on the internet to access it, while you, as an employee, will be able to access. Get someone on a completely different network somewhere on the internet to access the URL, preferably with Netscape or Internet Explorer and see what happens. HTML-browsers are a bit better at giving an understandable error message than WML browsers.

If others can reach the URL via Netscape or Internet Explorer, then there is most likely something wrong with the way WAP clients access the service. In the cases where it doesn't work, the user may be connected to a so called "walled garden" operator which means that the operator only allows its users to access a preset list of sites. If this is the case, the user should quickly switch to another operator - at least in my opinion. If you've narrowed it down to a problem with the operator or its WAP gateway, there's not much more you can do except get other users, connected to other operators and other WAP gateways. If it works for them, you are pretty sure of where the problem is. One word of caution - if you go as far as to contact the operator or the owner of the gateway, please please please be 100.00% sure that the problem isn't caused by anything on your side.