It is fair to say that she hadn't found her way in the role when the American critics saw her early in the run, or when I first saw her, which was about three months in. She - oddly - was *much* better after she was fired: I saw it again in the period in which she was working out her notice after The Unpleasantness, and the difference was like night and day. That second time, there was a go-for-broke quality to her performance that was missing the first time, and it made all the difference. If she'd found that performance before the critics came in, rather than just in the last couple of weeks of her tenure in the role, the whole saga might have played out quite differently.
...and having said that, my favourite Norma in the original London production was Rita Moreno.