I must have played Ludovico Einaudi’s song Una Mattina at least 25 times in the last month and it has never shown up on my On Repeat playlist. By contrast, I play some other tracks a few times and they score near the top of the playlist. This algorithm really puzzles me!
If you don't see your On Repeat updating properly first thing we recommend is logging out from all devices and then logging back in. Occasionally this is necessary to force a sync of info between your device and our account system.
In case you see no difference after that, try a clean reinstall of the Spotify app on your iPhone. This will remove corrupted data that could be causing the issue.
Keep us posted! We'll be happy to lend a hand if you need more help.