Check out this list: http://www.silexlabs.org/amfphp/documentation/troubleshooting-and-debugging-your-project/
The most common one I've had is if you have extra characters (even spaces or newlines) after the closing ?> tag of your php.
Also if there's an error in your code, and your php is printing that, that'll kill it as well.
Try calling it outside of flash first to make sure everything's working.