this is somehow resolved by deleting all the themes and upload it again. but some post that has picture around 5xx width still not showing as featured post.
i'm thinking that it has something to do with cache option?
It's not enough that it's around 500px, it must be atleast the content width to be shown, and at least container width to be shown as featured (span over the sidebar). More info here. and here's how to add smaller images.
