The 1030 doesn't appear to have a problem loading routes. If it did, there would be many threads about it (that aren't 1 year old).
The problem being discussed in this old thread appears to due to the course name being truncated.
All of the Garmins have this issue.
It's easy to avoid: make sure the names are unique in the first 15 letters.
I had the same issue. The route names were different. I also tried deleting my other saved routes, thinking I had too many already saved. But uploading them as a tcx file worked.
Thank you!
The Edge 1030 has a limited maximum number of routes allowed to be saved onto the device. Once you reach the limit, no more can be installed. Try deleting several (or all) of your routes and try again. Back up your routes onto a computer before deleting the whole list.
https://forums.garmin.com/sports-fitness/cycling/f/edge-1030-plus/237324/course-limit-on-1030-plus
“It is not possible to store more than 100 courses on the device.”