If certain calendar events are not appearing on the group space calendar element, it is likely because the event falls outside of the date range set for those elements. The calendar element on a group space home page can be set globally to display a specific number of days before and after the current date. This must be done by a site administrator and will affect all group spaces.
Go to People > Constituent Manager
Click Settings (upper-left-hand corner) > Global Settings
In the List View dropdown, select which option of days you would like the elements to display.