Yeah, we'll add this to our to do list. Facebook updates are frustrating for us too because there's no reason given for why a particular post won't go through. They either just succeed or fail, and there isn't a great way for us to follow-up on that.
We don't want to add a Facebook button to every fuel-up on your vehicle page, but we could probably add an option to the fuel-up editing page that would re-send the fuel-up info to Facebook.
In the meantime you might also try completely removing Fuelly from your list of apps at Facebook, and the resetting your Facebook connection here in your site settings. That usually gets updates posting again.