Not a Garmin user. But after downloading a route (they're always called cycle_travel.tcx) I always rename the TCX file. Just guessing, but the Garmin might not like multiple files with the same name.

Thanks John will try renaming.

If you save your route on cycle.travel before downloading, cycle.travel will give your .tcx the name that you saved the file with.

But if you can wait a week or so there'll be an easier way to transfer routes to your Garmin!