stiffymcspliff on "Problem with add_feed"

I'm building a plugin to provide XML feeds for my Roku channel.

I have the custom feeds working fine, but when I create new categories they work fine when I visit them in my browser, but when I use a 3rd party (XML validation service or Roku) I get a 404.

The initial 4 categories are working fine and work like the following:

myUrl.com/feed/customfeed/?cat=1
myUrl.com/feed/customfeed/?cat=2

Feeds 1 - 4 work fine, but 5 & 6 do not work other than if I view them in my browser.

Any ideas?