I’d guess that they have a rating system and prefer assigning request to helpers who reliably perform well… which might be why I didn’t get repeat requests after my initial fumbling.
Sure, and that's an important tool for improving quality in the face of different skill levels / levels of effort. But that's independent of investing a bit in improving the skill level across the helper base; as you experienced, it's something where experience helps, and if your first time in the real flow is with an actual request, there's no way to avoid the helper needing to deal with both helping and learning simultaneously.