Appoinment delete problems

Hi,
I have about 10 users and they share their calenders between them and make
appointments for Doctors. The permissions are good for everybody. Lately,
sometime if they open a occurence and make an appointment for themselves or
somebody else, it works fine. But sometimes when they open up a appointment,
they cannot make any changes; cannot delete or add anything. Once they click
to open the appointment, it open up an e-mail invite but the send button is
grayed out. You cannot add anything in the subject line either. Also, if you
look at the appointment it has a occurrence symbol on it but it has a line in
the middle. So, it is like a cancel ocurrence. Please help!
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FZ