Sample holder arm sticks in a closed position

Sometimes the sample holder arm sticks in a closed position on the CF/CFS57U. Here are some tips on how to fix it.

Loosen the sample holder thumbscrews and readjust it's position. Then retighten the thumbscrews, making sure that the silver retainer ring is not resting on the sensor port.
Make sure that the sample is flush with the sensor port when being presented for measurement.
If the above tips do not stop the sample holder arm from sticking, then the silver retainer ring may be bent, requiring repair of the instrument.