RSS issue

Hello,

I have been using your theme for a very long time and I have to say it’s great. But recently one of my visitors has sent me a message with a notice that there is some RSS problems.
“guid” tag value (unique) for the RSS item doesn’t change when post is updated (always stays same as the link to the post)RSS readers/services (those that
work properly), don’t show the updated RSS item. Once an item is “polled”/fetched by the reader/service, it’s not updated (and definitely not shown as unread if it was marked read previously) if the “guid” tag is the same (because it is telling that this RSS item is unique), even if other that RSS item contents change (e.g. “title”,
“description”, “content”).

Could you help me? Thanks.