This meter can have multiple issues related to measurement. If there is a problem, it typically has to do with the fact that the yarn is passing thru the meter but no measurement is occurring. This can be caused by 3 reasons:
The person is not inserting the yarn properly into the slot and the yarn is not actually resting in the pulley's groove. To ensure that it is going into the groove on the pulley, stretch the yarn between both hands and make it taut. Then as you have both hands straddle the meter, carefully insert the yarn into the slot and hopefully, it will go into the groove.
The pulley itself could be loose and it is moving in and out and, therefore, it is defective and needs to be replaced.
If the meter has been used extensively, fibers may have become wrapped around the pulley axle and the fiber is preventing the yarn from turning the pulley. The user needs to remove the small lid be removing 1 screw underneath and then they need to clean the area of the shaft behind the pulley. I recommend using a tweezer to grab the yarn and then pull it off.
A small internal black rubber part may have come off inside and is going under the pulley. Simply remove that rubber part and discard it. It is not critical.