In an ideal world you should use the HERE Map for Garmin Devices as slight differences between OSM and HERE maps could cause some minor issues on the Garmin.
Here maps do require a Gold account however.
Things to clarify as Nick mentioned:
Which Garmin Device do you have?
And also are you using a PC, Mac or MyRoute-App Mobile (iOS or Android) or even going directly to Routelab in a Browser, to transfer routes?