There are two different types of data logging. The one you are using must be the Aquasuite data logging that only functions with Aquasuite running to collect the data. What you need to do is use the Aquaero data logging.

In the Aquaero tab of Aquasuite there is a sub tab that lets you set up logging. Those logs can be set to record all the time. Plus once they have been running when you open the Aquasuite logging tab you can import the logs to display.

Aquasuite Data Logging

Here you can create your charts and save them but they will only show data from when Aquasuite was opened

Aquaero Data Logging

Here you can set logging on the device itself that runs full time. Then with the 'Synchronise log data with Aquasuite' button you can import these data sets into the Aquasuite data log charts and show data from before Aquasuite was opened.