When you view another firm member's calendar the appointment shows unavailable if you are not assigned to the associated file and\or contact or if you do not have access.
To fix the issue please have the user whose appointment it is make the following change in their preferences.
1. Select Office tab
2. Select Preferences top left
3. Select Calendar - Other
4. Select "Allow details of all my appointments to be viewed by...."
5. Add over the user who needs to see these appointments
6. Have the user who wishes access log out and back into Amicus