Yes, Cockatiels lay eggs every 2nd day. I'm not sure how often you are looking in the nest box, but I'm hoping its not more than once a day. If you disturb her too often, they eggs may be broken (or jostled enough that they don't hatch).
My thought is that maybe you looked shortly before she laid her previous eggs, so you thought they were laid the day after you looked, but this time you looked a little later & they had already laid the one for today.