Hi Butter Tray.
If you're referring to the tracks inside the Midi itself, my best guess is that the RGM program can only read the first 6, maybe first 8 tracks within a midi file. And if that IS the case, then you would have to simplify and reduce your midi to a set number of tracks with no extra or empty tracks.
I can't be sure if that's the case. I never asked any of the RGM experts if there was a special trick to the midi import. But I've seen different midi software, even my own Casio keyboard have trouble reading certain midis if they had too many tracks, or different tracks put in odd places. Sometimes putting the drums in the wrong track number can make them not show up or play incorrectly because certain tracks may have standard mapping defaults.
If that isn't the issue, however, then RGM may have certain bugs when it comes to Midi, which could also have issues with Windows 10, since the RGM software stopped development 10 years ago. So it would have been built using either Windows 7, or possibly Windows Vista. Without dedicated software, Windows 10 may reproduce midis in unexpected ways, using instrument sounds you aren't expecting it to use.