The one you failed was actually the Integrity vs. Integrity roll, which was also the one most likely to fail.
The roll scheme is, admittedly, upside-down, though the odds were correct. I'll fix that before the next one. You'll get the actual rules once Amu has figured it out in-story.
If you want a hint, it's... it could have gone worse, although 'worse' is in some ways 'better'.
Right, so these rolls - especially Overgrowth - weren't for bonuses, they were all for mitigating failures.
If they were rolled upside down, that means Overgrowth was rolled first. Which heavily implies that it's trying to train multiple things at the same time that is a problem. She isn't really training Integrity actively, because she doesn't need to - it already automatically strains Amu's own Integrity to try Solar-speed training multiple things like Dreamwalking and Lore, so that the act of training multiple skills/abilities at once can also train Integrity if we voted to do so.
It's just not healthy for her to do it, as I'm guessing it could be building up her equivalent of Limit while she does.
Guessing the reason Illusion didn't get a roll is either because cloning textbooks falls under Miki's drawing specialty and doesn't unduly strain her, due to getting natural help from the Chara, or because she's allowed to train 1 thing without straining and Illusion got picked (possibly because it was the least strenuous of the lot due to the former reason).
So Amu doesn't -really- get natural Solar training times, she gets a knockoff version powered by Dia (or the "Chara system" in general) that allows her to train maybe 1 thing at a time at Solar training times without issue, or else relies on needing everything to be closely tied to her Charas to not strain her - i.e. she could probably train Athletics, Performance and Craft at the same time if she tapped into Ran's specialty, Miki's musical specialty and did cooking/sewing for Craft to leverage Su's specialty.
But trying to learn both Teleportation and Thermokinesis at the same time is virtually guaranteed to strain her as none of her Charas have any apparent way to use their specialty to offset the load.
@#$% me.
.......That really sucks.