The function comment_reply_link echos the function get_comment_reply_link. If you look at the source code you see, that up to 3 arguments are needed for a proper functionality.
$argsan array of optional arguments that overwrite the default values$commentan optionalcomment_idor WP_Comment object the comment should reply to.$postan optionalpost_idor WP_Post object where the comment is going to be displayed on.
If the function returns null, it cannot figure out the relating $post. Try to add this parameter to your function call, e.g. comment_reply_link(null, null, $post_id) or $comm.= get_comment_reply_link(null, $comment); (within your foreach loop).
If the function returns false, comments are disabled for this post.
Source: developer.wordpress.org