Why isn't Google Reader able to detect the feed for a website?

Question

I have two websites that declare an RSS feed, like so:

<link rel="alternate" type="application/rss+xml" title="Awesome Site &raquo; Feed" href="http://url.to/valid/feed" />

Both sites run WordPress, but the automatic feed link injection has been disabled for the custom theme.

One site can be added to Google Reader by entering just the domain name; the other cannot be added without giving Google Reader the precise address of the feed.

I'm at a loss as to how this is even possible.

Answer 1

Are both feeds outputting valid XML?

Answer 2

Google caches feeds in Reader. Its exact recrawl interval is unknown, but after a week or so, the issue resolved itself when Google recrawled the feed.