maybe you should try sth. like this:
...
add_filter('comment_text', 'my_filter_function',10,2);
...
After defining filter and function, you should specify priority (by default 10, and number of accepted argument (by default 1).
accepted_args is an optional integer argument defining how many arguments your function can accept (default 1), useful because some hooks can pass more than one argument to your function.
