Do you see <?php phpinfo() ?> on your broswer? Or do you see nothing? If you see nothing your provider (I'm assuming you are running this on a hosted machine) has turned off access to phpinfo() because that information can be used for bad things.
If you are seeing the code then you should check the extension of your files. The ones that work probably have a different extension than the ones that aren't. Apache uses the extensions to determine what application handler to put the file through. Current versions of php use .php, older ones used .php3 and other such things. And your host could be using whatever they want I've used .html to force php parsing on all files from some web servers before (I only did this on sites that were completely built using PHP and had no static HTML pages).
