The machine is supposed to have a large metal container outside the flap, to receive the mix. The machine would only keep the flap open if it detected this metal. But they were using a plastic container, so they fooled the sensor with a metal clipboard. This must have become dislodged, do the flap shut.
Methinks: If it is meant to be used only with a large metal container, only use it with a large metal container. Full stop.
If it must be used with a plastic container, change it so that it will work with a plastic container. Change it, not just fool the sensor.
Is this not obvious?