If users are not able to see a particular time off in the time off calender, even though they have already booked it, then enable the option 'Display on Calender' in the time off type settings.

Follow the steps given below to have that enabled :
– Go to Administration –> Time Off Setup –> Time Off Types
– Click on the Edit button next the Time off type that you want to modify
– Put a check mark to the option 'Display On Calendar'
– Click on Save.

Now the users should be able to see this time off type on the shared time off calender.