Hey there, @Michael52873 - great question!
For context, Calendly will display the first 10 sub calendars that Microsoft sends us in the check for conflicts and add to calendar sections (please see below):
If you have more than 10 shared/sub-calendars, that may be why you are not seeing your preferred calendar(s).
Are you able to temporarily remove some of the calendars you currently have subscribed to see if that switches which calendars display in Calendly? You may need to disconnect + reconnect your calendar from your Calendar Connections page to update the display. Alternatively, some instances of O365/Outlook calendars will allow you to drag and drop your calendar list to rearrange it. Could you give this a try from your calendar, then refresh your connection within Calendly on the Calendar Connections page?
Once you are able to view the correct sub calendars, you can select up to four sub calendars to be checked for conflicts.