It was the power of love. Bullshit, I know, but thats what it was.
When she remembers, on her deathbed that Suzaku is Japanese, she immediately says "no, I can't, I can't think of that, I can't," and she resists the Geass.
It wasn't because she was unable, she actively resisted because it was important to her, and she won. Of course, Lelouch assumes it is impossible to completely resist the Geass, but he was not witness to Euphie's last moments, so he doesn't know that he is wrong.
This little caveat regarding his power IS going to come back and bite him in the ass eventually. Count on it.