I only see points; where are the polygons?
Zoom the map in. Polygons are not shown at zoom levels 5 and below due because the complexity may not be representable within the available pixels without simplifying the shapes.
Some geographic features are missing polygons.
Multi-geometry. The ten largest-area polygons found in a multi-geometry placemark will be displayed on the map. Remaining polygons will not be displayed. Break multi-geometry placemarks into multiple rows if needed.
KML is too big. A cell of data in Fusion Tables supports a maximum of 1 million characters; if you have more than this, you need to simplify the polygon or line or reduce the precision of coordinates in order to fit the description data into the 1 million character limit.
A few polygons disappear as I zoom in or out.
Confirm the polygon is described as a counter-clockwise list of points, per the KML standard. Not all KML generation software does this correctly. Fusion Tables does a best-effort to display the polygons, but this will sometimes fail at particular zoom levels depending on how the shape falls across map tile boundaries.