My guest was unable to book my calendar despite it being free

There are two possible reasons when this happens. You may have given your assistant other rules that override what is considered free time.

Check the calendars you’ve allowed your assistant to access. If your colleagues’ or spouse’s calendars are shared with yours, your assistant wouldn’t know which calendar you want it to check against. Go to General Configuration → Select Calendars to Check for Conflicts.

Another area to review is General Configuration → Scheduling Rules. There is a feature where you can set the maximum days in the future (for example, if you don’t want someone booking you six months from now) and the minimum notice (to avoid last-minute surprises).