If you haven't made recent entries, you may log in to your Diary app and find that your older "Medication Taken" entries do not appear on your Dashboard widget. Fear not! Your entries are still there. As of updates in early 2017, the "Medication Taken" widget only shows the last seven days worth of entries.
To see your older entries, click on the cog icon at top right of the widget. Then, click on "Show All Data". Your older entries appear here. Click on each to view details, edit or delete.