The spring that holds the cover to the left may be missing or disconnected, it is located on the back of the coin comparitor.
The sliding cover is spring-loaded, you can move it to the right to remove or install a sample coin, but the cover should move back to the left fully so it covers as much of the sample coin as possible. That sliding cover is part of the sensor for the coin comparitor, must be over the sample coin. The round sliding cover that has the arrow on it should slide to the left to cover most of the coin. From your photo it looks like you do not have the sample coin installed properly in the coin comparitor.