Knock function is still being optimized. Knock itself works well when the app is working in the background. However, iOS will suspend the Sesame app after it has been idling for a while. In this case, you will have to wait for a couple seconds when you get back within bluetooth range for Sesame to "wake" the app.
The time it takes for your Sesame to awake the app is strictly determined by iOS, so it might take a few seconds longer for some than other. We are confident that this can be solved and released in following two weeks.