Why doesn't the i7000 press down?

Why doesn't the i7000 press down?

The order of checks is:

1. Check 110V or 220V power supply

2. The grating is intruded or the transmitting and receiving electric eyes are not aligned, and the electric eyes will display red

3. The emergency button is pressed, of course it won't come down

4. The board under test is not placed in the position, and the anti-fooling circuit of the fixture works

5. Check whether the air pressure of the air supply is 5kg/cm2?