Not all shared calendars visible

Not all shared calendars visible

Hi,
 
I have shared my calendars with some colleagues but the last colleague I added cannot see the calendars. He is the 6th person to be added.  Is this a bug or is there a limitation on sharing calendars for free? (We are a not for profit charity and I wanted to share a calendar  around a maximum of about 20 people)
 
Thank you.