This may be something nobody supports, but here goes :")

When a cub fails training and a Grandpaw is active on your account, there is a chance that your Grandpaw will give the cub a pep talk, which always seems to result in the cub wanting to try again.
What if they do actually get another chance that day? I don't think it would break the game as the peptalks are rare occurances, even when the cub fails training, so it's not like every single cub would get peptalks until they succeed. By some odd RNG quirk, if it did happen that one cub received multiple peptalks, they can be limited to 2 tries per day.
Once they are given the peptalk, they can be placed at the back of the line so the other cubs can have their go before the cub in question tries again.

There's no real reason for this suggestion - I don't find training difficult, I just noticed that right now, it's just flavour text and thought this might be a good detail to fit it