I am not sure whether I am missing something here, or could/should have done something.

Mild spoiler:

In the quest 'Woe to the Vanquised', if one talks to the two looting soldiers and chooses the first option, Gerald starts to cast a sign, Axii. The first time it failed, the soldiers killed the civilians, and I could not interfere.
I can cast Axii, so I wondered why it failed and reloaded. The second time it succeeded, but as far as I can tell, I did nothing different.

Is there something I should have done the first time and accidentally did the second time, that caused the Axii to fail/succeed?

If not, how does the game decide whether Gerald succeed in casting the Axii, is it by pure chance?

Thanks for the answer. I liked Witcher 1, but the barely useful tutorial of Witcher 2 did already disappoint me, and failing quests by pure chance, without anything I could have done to improve the chances, is disappointing.

I am ok with skill tests if I had a say in how good or bad I am, by choosing the kind of build or whatever. But this is the beginning of the game, no opportunity to train anything yet, at least none that I noticed.

And the game did not even tell me what just happened. Sure, 'Axii failed', but not why, or how. Gerald just stood there and watched two soldiers slaughtering innocent civilians.