Bookings calendar not pushing to outlook calendar

We have booked something on bookings and it does not show on outlook. This is weird, because it is only happening every once in a while for absolutely no reason. Today, we got an email saying the appointment was cancelled. But, in fact, it was still listed in bookings as an active appointment but it was no longer on outlook. Is there a reason for this? This person it is happening to has appointments regularly. This is randomly happening and only to her. Wondering if this is a setting I need to change?