The code you're using is a bit out of step with that Feedburner is expecting.
What you want is <wfw:commentRss> (case-sensitive) which is supposed to point to the comments feed of the post.
Pair that with the <slash:comments> tag right before the closing </item> tag for a pair like this:
<wfw:commentRss>http://example.com/path/to/comments-feed/</wfw:commentRss><slash:comments>13</slash:comments>The comment count will be taken at a snapshot of whenever Feedburner creates the cache of your feed so may not be exact when it appears in the feed reader.
