Ok, yes it definitely sounds like that smoke detector sensing element is likely bad. Is it visibly dusty?
From the manual:
Clean the alarm once a year. To clean the alarm, remove it from the mounting base. You can clean the interior using compressed air or a vacuum cleaner.
Blow or vacuum through the openings around the perimeter of the alarm. The outside of the alarm can be wiped with a
damp cloth.
After cleaning, test the alarm by pushing the Test button. If cleaning does not restore the alarm to normal operation you need to replace the alarm.