Absence days for open ended sicknesses are created via the Snapshot batch job which should run overnight. If this job fails to run, your open ended sicknesses will not generate absence days.
Check that the snapshot job is running
In the Admin Tool, then click Batch Jobs.
Scroll down to and single click System - Update Employee Snapshot.
On the right, click View the log for the selected batch job.
On the right, click View workflow log.
If the batch job is running with an error message, please search the error in the knowledge base to find an article.
If snapshot is not attempting to run, please raise a case with support.