This could happen for a variety of reasons, but the most common reason is that your feed lacks GUIDs, and your items all share the same URL. This is technically a valid feed, but it will have problems.

An Item's GUID is a unique identifier that we (and other services) use to identify the item as having been seen (or not). If the item contains no GUID, we have to use the URL instead. Since your URL is the same every time you post, we can only determine that there are no new items to post.

There are three ways you can resolve this:

1. Add GUID fields to your RSS feed with a unique value for each item when it is posted.
2. Change the item's URL when it is posted (e.g. by linking to an individual post instead of your main page)
3. Edit your dlvr.it settings and enable "Re-post items with updated timestamps" on the Advanced tab of your Feed settings. This will force dlvr.it to consider the timestamps, and it will re-post an item whenever the timestamp changes. Warning: This can cause duplicate updates if the timestamp updates without item content actually changing, however.

What is a GUID? (source)

Stands for Globally Unique Identifier. Sets a string that uniquely identifies an RSS item. If this tag is used, a feed reader may use the string to determine if the RSS item is new.

Each GUID must be unique and it is up to the webmaster or whoever maintains the RSS file to make sure of this for every GUID that is created.

A GUID could be a URL pointing to the RSS item content, or not.

The GUID for a given item should not change once it has been assigned.