The service area polygons depend on how much the road network was traversed. So bring up the service area layer properties on the Line Generation tab, turn on Generate Lines and then turn on the Split lines at breaks. Solve it again. And now you can see how the lines are being traversed and what break values are present on them. Note: you may want to symbolize the lines on break values to see where they start/end. Examine the data to see why the final break value is not being reached. It is likely there is some restriction or disconnected edge that is not being traversed.