My 15 yr old is having seizures but eeg was normal? just wondering how can this be?

Imagine : Imagine if a bank had a security camera that was only on 20 minutes a day. It would be very unlikely to catch a bank robbery, and might not even be able to detect the clues a bank robber leaves behind. A routine eeg lasts only 20 minutes. Unless something suspicious happens during the time the eeg is running, everything may appear quite normal. There are ways to increase the chances that an eeg will find something abnormal. These include asking the subject to breathe heavily and deeply for 3 minutes, flashing a light in the subject's eyes, performing the eeg on a sleep-deprived subject, doing repeat eegs, or increasing the length of the recording. Sometimes behavior that appears outwardly to be a seizure is captured during an eeg recording, but the eeg doesn't show brain wave abnormalities. Such a situation may indicate that the subject has nonepileptic seizures.