iStudiez Pro can show only those events, that are visible in your Calendar app.
The app itself does not contain or store your Calendar Events.
If some of your Calendar Events are not shown in iStudiez Pro, check if you can see them in your Calendar app.
In case you don't see your past Calendar Events (e.g. those that were a month or two ago) in Calendar app, make sure it is set to show all events.
For example, when on iOS - go to System Settings app > Calendars > Sync > make sure you have "All Events" option checked there.
Otherwise, you won't see those Calendar Events in iStudiez Pro as well.