It took me several days to finally figure this out, but I couldn't get feeds to work in iTunes unless the site had clean URLs turned on. iTunes would say that "there are no playable episodes" and "the URL might point to text-only episodes, or contain file types that iTunes cannot play". You may wonder why I wouldn't use clean URLs, but my school's PHP server for personal sites apparently doesn't support it. I think it'd be nice in the future if this could work for anyone who for some reason isn't using clean URLs.