After successfully unlocking your door by knocking 3 times, this function will temporary deactivate. This is to prevent Sesame from "accidentally" unlocking itself if you throw your phone on the bed or couch when you get home. All you have to do is enter the app screen to "refresh" it There is no need to readjust any setting.
Another thing to note is that there can only be a time lag of 5 minuter or less between opening the app and using the "knock to unlock" function. In other words, the "knock to unlock" function works only when you have opened the app 5 minutes prior to using it. This is also to ensure that you do not unlock your door if you accidentally knock on your phone.
Troubleshooting: How do I set up the Knock function and how does it work?