AWS http static page of one EC2 instance always jump to another one

I have setup different instances on AWS, and the problem goes:

1. I setup one html page (Wordpress) on one EC2, say, a.com
2. I cloned that instance into another EC2, then start changing the settings and put b.com there.
3. On route 53, a.com and b.com domain names are pointed correctly to their IP and endpoint.
4. when I visit b.com on browser, it ALWAYS jumps to a.com, no matter what ....

It's persistent and I've tried many things and still can't find the cause. Hope posting here can get some hints. Thanks!