RSS? Updates to post not in feed.

The posts I put in my blog are for users to comment on and are informational post. occasionally I update them with new information.
I use bloglines and it updates before I checked it. However, posts with additional info do not show in the rss feed like a brand new post does. I thought with this update the reader would catch it but it didn;t. since this post had been in the reader once before I thought it might not catch it. I would like to force the readers to catch the updates. Is this possible and if so how do I do it.