What is wrong with my RSS feed?

How am I supposed to know? I bet you Feed Validator will. Great tool to validate your RSS feeds.

To use it, simply enter the address of your feed and click Validate. If the validator finds any problems in your feed, it will give you messages for each type of problem and highlight where the problem first occurs in your feed. If you're unsure what a message means, click the "help" link next to the message for a fuller explanation.

Link:
http://feedvalidator.org