The issue:
When you open the Appointment Book it has no Fill Gap icon in the obviously available gaps:
The reason:
EXACT is configured to not offer gaps within a period (2 hours) of the current time, because it is unlikely that patients will be available at such short notice.
When you open the Appointment Book, it is by default at the current time, so there are no patient gaps available for another 2 hours, and hence you cannot access any tooltip suggestions.
To check, go ahead more than two hours and you should see Fill Gap icons in the gaps.
The workaround:
If you want to offer gaps within two hours of present, you can still manually contact patients from the Short Notice List: