When you start an import the plugin will start importing in the background. Eventually, if you’re importing a large enough amount of data, it will be rate limited and will be forced to continue the next day. The continuation is done through a scheduled task.
In order for the scheduled task to run you first need to have cron archiving setup. This is where scheduled tasks are initiated.
Once you have cron archiving setup, use the TasksTimetable plugin to determine when the scheduled tasks will next run (the relevant tasks will have
Piwik\Plugins\GoogleAnalyticsImporter\Tasks in the name).
If these tasks never run, even if they are listed, then something is failing in the cron archiving command. The next step to debug is to check the output of the command.