Messages don't automatically delete even after setting it to delete after 30 days.

I'm trying to use the feature that automatically deletes messages after 30 days but the messages don't disappear. It asked me if I wanted to delete these messages and I said yes, when I went to check, all the messages were still there. How do I delete these messages? When I try to delete them one by one, the app locks after a while, I have to restart the phone to get it to work again, and then, when I open the messages app again, they are still there.

This is on an iPhone 4s running 8.1.1