My read posts are showing up as unread. [View all]

I just purchased a new Dell laptop. My last one had seen the end of its useful days. But "Here's the Deal".

I go on DU and read the posts. The lettering is blue before I read them and turns to a purple shade after I read them. This is certainly helpful, so we don't keep reading the same posts over and over.

But.... with my new laptop, while the post turns to the purple shade after I read them, if leave DU, when I come back to DU all the "read posts" have turned back to blue. So I can't tell which ones I've already read.

Is this happening to anyone else? DO I have to switch something on in DU to keep read posts from showing up again as unread?

Thanks.