When calculating route shapes along a road network, Network Analyst may not successfully calculate a route connecting the stops for some shapes. In these cases, since network shapes could not be created, the tool creates straight-line shapes instead.
If straight-line shapes are not adequate for your needs, examine your network dataset and the stops associated with the failed shapes, make any necessary updates, and run the tool again. Failed route solves are often caused by a poor quality network dataset with disconnected areas or by transit stops that are in the wrong location. You may also need to adjust the restrictions used in your travel mode, or experiment with the bearing and bearing tolerance tool parameters.