If a chart you're trying to post isn't showing up at the top of the feed, first - have you shared this particular chart to the Community before? If so, it's actually not currently possible to re-share a chart to the Community. We chose to allow everyone's chart to appear at the top only once so that everyone would have a chance to appear at the top. Sorry about that!
However, we realize this isn't the most convenient option, since re-shared charts don't get the same attention they deserve. We're working on adding smaller groups and other options to make updated charts more visible. So, please stay tuned for that. In the meantime, if you leave a comment on your chart, a notification will be sent to everyone who's commented on that particular chart. So, it's possible you could get some feedback that way.