In some units, the holes in the outer plastic housing do not perfectly line up with the holes in the inner plastic molding. This can cause sensors and/or the stylus to be difficult to connect or disconnect.
For sensors, try pinching the release catch while wiggling the sensor and pulling out.
For stylus, you may need to apply outward pressure to the stylus using a hard metal object such as a coin or a car key.
Note: This problem usually goes away after sliding the sensor or stylus in and out a few times. If it does not, we can replace the unit.