Please check the Limitations of KML/KMZ on Google Maps API:
Google Maps supports the following KML elements (that is, KML queries in the Search box and GGeoXml objects in the Maps API):
Placemarks
Icons
Folders
Descriptive HTML
Entity replacement via <BalloonStyle> and <text>
KMZ (compressed KML, including attached images)
Polylines and polygons
Styles for polylines and polygons, including color, fill, and opacity
Network links to import data dynamically
Ground overlays and screen overlays
This list does not apply to Google Earth's export to Google Maps, My Maps import, or My Maps export to Google Earth.
The KML parser generally silently ignores XML tags it does not understand.
Google Maps currently has specific limitations to the size and complexity of loaded KML files. Below is a summary of the current limits:
Note: these limits are temporary and are subject to change at any time.
Maximum fetched file size (raw KML, raw GeoRSS, or compressed KMZ) 3MB
Maximum uncompressed KML file size 10MB
Maximum number of Network Links 10
Maximum number of total document-wide features 1,000
You cannot extract data from Google Maps.
What you could do, is to get the data from other sources. Depending on your Area of interest, the boundary might be available from your local government agency, or even OpenStreetmap.
Best Answer
For such conversion I often use the free and opensource tool ogr2ogr
An easy to use command line. A cheatsheet with must common command from Boston GIS
It would be something like: ogr2ogr "KML" output_filename.kml input_filename.shp
Maybe this conversion is more accurate. I hope it helps.