HELP: function add_filter works at local, does't work on server

I’m writing a plugin with similar following codes:

add_filter( 'the_content' , 'my_function' );
function my_function( $content ) {
	$test_config['timeframe'] = '7d';
	update_option('test_config',$test_config);
	return $content;
}

It works well on my computer local test, but when I upload it to the server, it refuse to work.

I tried add_action(‘wp_head’, ‘my_function’), it works.

Any help?

Thanks.