As Ken said, not all (in my experience, few) alerts have the polygon data for the map. If you want to see what sort of alerts have them go to alerts.weather.gov, scroll down, and select the "National" (first) link in the list. If you see a Flood Warning, those always (in my experience) have a polygon. Select it, and then on the page that displays the warning, do a View Source for that page. Scroll down and you should eventually see something like this:
<polygon>33,-92.1 33.11,-92.17 33.18,-92.12 33.07,-92.11 33,-92.05 33,-92.1</polygon>
If an alert does not have a polygon, there are no numbers between the <> , </>.
For testing purposes, you can add the county or zone code of one of these active alerts to your settings file. Look a little further down in the View Source page for "UGC" and use one of those codes.
If that isn't making sense, let me know. I was neck deep in this for a while.