To improve feed fetch time, those with invalid URLs or that haven't been updated in more than 6 months are no longer checked. However in some cases feeds will be silent for a long time and then pick up.
FeedsAnywhere tries to get around this problem by checking any feed that is viewed individually, but a bug existed that wouldn't add new items more than 30 days old. That is now fixed. Once a new item is found the feed is added back into the regular rotation and its 6 month clock starts over again.