When I went to physical therapy for my neck pain, they put a plastic device over my head to measure my neck motion. The device shows that my motion is improving but I don't feel any better. Is it possible the neck piece isn't working right?
You may be referring to a tool called an inclinometer. This device sits over the head like a baseball cap. It has a special nose piece to hold it in place for accurate measurements. Velcro straps adjust it to your head size and help hold it in place.

The cervical spine inclinometer measures neck range of motion. There are six possible motions. These include forward flexion (chin to chest), extension, rotation to the right and left, and side bending to the right and left.

The therapists hands are free to guide your movements and record the results. These advantages of the inclinometer reduce the chances of error. Usually you are sitting when the measurements are taken.

It's likely that your motion has improved but there seems to be a lag in your perception of the change(s). Ask your therapist about this. He or she can use some of your treatment time to help you regain this lost sense of joint position called proprioception. There may be other reasons why you don't feel better. Bring this to your therapist's attention for further testing and discussion.