I am solving a service area polygon with a roads network dataset and attached is the result. Breaks - 250, 500, 750, 1000 miles. The polygons in the image are only drawn north and south of the facility.
Polygons are drawn incompletely for certain central geographic regions, and are very poor for coastal cities. There are regions with huge gaps in polygon fill in. There are clearly roads bounded on all sides for these large areas, so I???m not sure why the graphic cannot show this.
This kind of results when testing with Esri street data too.
Service Area polygons are drawn around the edges that can be reached within the specified break values. You are using Breaks - 250, 500, 750, 1000 miles but the line deep purple lines (I presume that this is the service area lines) show that not much of the network was reached. So you most likely have disconnected portions of your network that is preventing the service area to reach up to the 1000 miles towards the East. That is why you are getting only an elongated north south service area polygons. Note that the "disconnected" portions may be caused by topology issues or restrictions.