Ah.. *changes* to entries is a different matter entirely (compared to *new* entries).
We only process new items when either a) their GUID (Globally Unique ID) changes, or b) their title or destination URL changes. Items are not changed if only, say, their description changes. The reason for this is that otherwise it would be necessary for us to check every item of every feed for minor changes (and we process tens of thousands of items per minute), whereas we use the GUID of each item as a marker of uniqueness which is a lot faster. So.. if the GUID if your item changes, your item is updated. If it does not, it does not. For items without GUIDs, we use title/destination URL as the equivalent.
Separate to all of this, I can entirely wipe your feed's content and recrawl it from new, but this is a manual operation and not one that would be useful over time.
This "feature" is one that a very small percentage of users find extremely restrictive, but which most are not concerned about, so unfortunately it's a trade off (for us) of having a very fast, reliable service versus providing a feature not many users need. If you do need it though, we might not be the ideal solution (yet).