This is a limitation of Google Calendar. Events older than 12 month are usually only available online in the web browser at Google Calendar

Since aCalendar is not involved in synchronization, we have no influence on this. 

Ideally contact Google about it, the more people contact Google, the more likely it is, that this limitation will be lifted.

Workaround (past events only):

To manually synchronize old events to Android, you can use the following workaround (needs to be repeated every 12 months):

  1. Go to Google Calendar on a PC in a web browser
  2. export your calendars (Export your Google calendar)
  3. re-import the calendars (needs to be unzipped before) (Import events to Google Calendar)

The re-import will update the modified timestamp which is used by Google to determine if the event should be synced to mobile devices or not.

We are currently working on a solution to fully automate this Workaround (planned for version 2.3).

Also, we are working on a solution to make Google sync future events further than 12 months away (planned for version 2.3 as well).