The problem with door knock detection is that the acoustics vary so much from door to door it can be very difficult to get it right.
Lockitron had a knock detector on their first generation lock and dropped it for the second because it was just too tricky.
http://help.lockitron.com/article/95-whats-bolt-and-how-will-my-lockitron-order-change
Removed Knock Sensor
We found that certain doors would trigger the knock sensor too easily, thus keeping the crowdfunded Lockitron awake and draining batteries while other doors would require significant knocking before the sensor triggered. The knock sensor functionality in the crowdfunded Lockitron requires a very special, low-power electronics circuit that is not adjustable in the field. Given that we already set our knock sensor to be as sensitive as possible, we decided that we unfortunately cannot offer consistent knock sensing capabilities and are removing this feature for now.
If it works for you, great, but if it doesn’t, it’s not surprising.
A more consistently reliable option for visitor detection is a smart Welcome Mat. 