Per the WordPress Codex on plugin development, on File Headers:
The top of your Plugin's main PHP file
must contain a standard Plugin
information header. This header lets
WordPress recognize that your Plugin
exists, add it to the Plugin
management screen so it can be
activated, load it, and run its
functions; without the header, your
Plugin will never be activated and
will never run. Here is the header
Plugin Name: Name Of The Plugin
Plugin URI: http://URI_Of_Page_Describing_Plugin_and_Updates
Description: A brief description of the Plugin.
Version: The Plugin's Version Number, e.g.: 1.0
Author: Name Of The Plugin Author
Author URI: http://URI_Of_The_Plugin_Author
License: A "Slug" license name e.g. GPL2
The minimum information WordPress
needs to recognize your Plugin is the
Plugin Name line.
If I had to guess, I'd say the problem is that the
Plugin Name header is missing from the top of the plugin's main file. Not knowing what plugins you're using, this is the most complete answer I can give right now.
I should add that in order for a file to be checked for headers, it must be a
.php file either in the plugins directory or in a subdirectory of the plugins directory. So, for example,
wp-content/plugins/my-plugin/plugin.php would be valid, but
wp-content/plugins/my-plugin/lib/file.php would not.