Sometimes that behavior in appointments here but not there can be "fixed" by running the Failed Appointments Report. It's worth a try if you have not done so in a long time.
To help answer your question I need a little more info:
Where in the phone center are you seeing it? Under the next exam column at the bottom or under the scheduling button or somewhere else? Is it a past date or a future date?
Apparently EVERYONE in our office is blind, except one person who found the appointment on the scheduler. He was scheduled with the wrong doctor and in the wrong room so nobody saw it there.
Problem solved! Sorry!