aCalendar is not involved in synchronization, it's just a user interface to the Android calendar storage.
Synchronization is always provided by your chosen cloud service.
The synchronization option within aCalendar tells Android to tell all cloud calendar services to sync. Your cloud service however should listen to database changes and sync automatically. If not, most likely some battery saving options block it from doing so.
If you have no synchronization between 2 Android devices (e.g. phone and tablet), you should first identify which device is the problem.
Test each device individually to the cloud (web interface of your cloud service, e.g. Google Calendar in a web browser).
Then check Android settings - accounts - your cloud account. (also linked from the menu in the calendar list, the calendar list also already shows basic sync info).
Check you global sync settings, account sync settings, (background) data settings and energy saving options.
Also see the service-specific help pages here and the help/support from your cloud provider.
For Google Sync Issues please see: Problems with Google Calendar Synchronization
For Exchange Sync Issues please see: Microsoft Exchange (hotmail, Microsoft 365, Office 365, live.com, outlook.com)