Hey are tricky things to install. Not worth the hassle, really but fun to see the speed/RPM.

There are a few things you can do, well that worked for me.

Get the sensor as close as possible as it works on reflection of an IR beam.
Make sure it's working, cameras can pick up the red dot. When I installed mine I just used my phone camera.
Make sure the sensor is the right way up. Might sound stupid but the correct orientation of the PCB will help. That is just a guessing game.

Good luck.