Symptoms on hosts stay active

We are currently using vRealize Operations Manager 8.0 and we have the problem in our environment that many symptoms on every host stay active although the problems are fixed. I assumed that the symptoms will change to inactive automatically but they won't. The symptoms I'm talking about have mainly the Event Type Notification.

Is there a way to change those symptoms manually? Or can someone explain why the symptoms don't change their states automatically?

Thanks in advance!

Regards,

Johannes