So I just had an interesting problem where a Blogger/Blogspot blog I help maintain couldn’t detect the RSS feed of a WordPress blog I help maintain.
The error message was “Could not detect a feed for this URL”.
It turns out the problem was two-fold. First: One of the files in the WordPress theme added an empty line at the top of the RSS feed.
Second: I had to update a post on the WordPress blog because *something* in the entire chain was caching the RSS feed!
So, if you have the same problem, run your RSS file through a validator, check if there are any leading empty lines, and make sure to update some post to regenerate the feed to avoid caching.