When you add a new feed to dlvr.it, you select when dlvr.it will make the first post to Twitter or Facebook. If dlvr.it doesn't post to your Twitter or Facebook account after you add the feed, usually this is because you selected 'Post next time a new item is created' option. if this is the case, dlvr.it will post the next time you create a new item.
See below for details and how to have dlvr.it post the old items in your feed.
From the Feed Details tab:
When should we make your first post?
1) Post the most recent item now.
-- dlvr.it will retrieve your feed and post the most recent item to your social networks now. dlvr.it will post again the next time you add a new item to your feed.
-- This is the default setting.
2) Post the next time a new item is created
-- dlvr.it will not post anything now. dlvr.it will post the next time you add a new item to your feed.
-- If you add a new feed and nothing posts, this is typically the setting you've chosen during the feed set up process. If you want to post items in your feed, you'll need to delete the feed from your dlvr.it Route, then add it back again -- and make sure you've selected a different option for this setting prior to saving the feed.
3) Post all items in feed (up to maximum)
-- dlvr.it will retrieve your feed and post the most recent items in your feed - up to the 'Maximum Number of items to post per Update Period' you set in your Feed Update tab settings. Then dlvr.it will post again the next time you add a new item to your feed.
-- The default 'Maximum Number of items to post per Update Period' is 10. You can adjust this higher or lower in the "Feed Update" tab.