It may be best to turn off your feedburner plugin that redirects your feed to feedburner. From there you'll be able to see if you're actual feed is up to date and if there's any difference between your feed and feedburner's. If there is, it's usually just a matter of updating your feed once you log into feedburner or an invalid character will sometimes throw it off. To find out if it's a character, try validating your feed - http://validator.w3.org/feed/ - if it's not valid it usually will give you an idea on what line number it's failing, which then should tell you which post it's having a difficult time with.