Problem: It doesn't seem to work very reliably
The knock sensor picks up any vibration that's strong enough. That means things like loose drawer pulls, wobbly drawers, and loose items lying on top of the desk can inadvertently set off the knock sensor. Sitting down a glass, slamming a door, and even loud music can trigger a knock. The knock surface should also be secure and firm. If it visibly moves when you knock it can trigger phantom knocks. Either improve the environment around the knocks ensor, move it to quieter place, or see "The Knock Sensor Is Too Sensitive" for other ways to work around it.
Alternately the sensor might not be sensitive enough! See "The Knock Sensor Is Not Sensitive Enough" for other ways to work around it.
How do you tell the difference? Knock on your surface while looking at the knock sensor. The red light will blink each time it hears a knock.
Problem: The red LED blinks constantly or randomly.
Solution 2: This can rarely be caused by a noisy power supply or external electronic noise near the sensor. Try a regulated power supply and move any high-current devices (electric heaters, refrigerators, air conditioners, etc) away from the sensor.
Problem: The knock sensor isn't sensitive enough!
Solution 2: Use a more sensitive piezo element. A 3/4" element will often more sensitive. (Avoid larger, louder elements as they include driver circuitry which makes them poor sensors.)
Problem: The knock sensor is too sensitive!
Upload the modified sketch to the Trinket after you make the change. You might need to temporarily disconnect the connection at pin #3 for it to program correctly.
Solution 2: Try a smaller piezo element. 1/2" (12mm) elements are about as small as you want to go.