No, she is not right, she should be able to see the spot the egg came out of, NOT the egg.

I have people on here regularly who have a vaginal ultrasound for IUI or for timed intercourse, and are told "oh no it's too late you already ovulated" because the doctor sees the place where the egg was. Not the egg, the hole the egg came out of.

Here is an article that explains that the corpus luteum is visible on ultrasound and ranges from 2-5 cm https://radiopaedia.org/articles/corpus-luteum?lang=us

Here is a study done in 1980 (that is, of course, nearly 40 years ago) and at that point in time, 40 years ago, they could determine ovulation had happened 80% of the time. Our skills and technology have improved hugely since then. https://www.ncbi.nlm.nih.gov/pubmed/7409241

Your doctor made a mistake. Doctors do make mistakes sometimes, it happens quite a bit really (too often in my opinion). But that is the only explanation - she missed something that she should have seen, whether it was a follicle or the corpus luteum (that develops after ovulation)