Your Garmin help, please

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I don't think that you can send a gpx route. You can send a gpx consisting of numerous waypoints which the nuvi will translate into POI's.
(Points Of Interest )

If it is a gpx track or route file you are sending it may consist of thousands of track points which the nuvi may be trying to translate to poi's. Rather than the tens that would normally be in a poi category.

This might be why some of the gpx files are failing but some work. Stupid bloody devices.