There are sveral possible reasons:

The dust tray may be full. Empty it.

The dust tray may not be attached properly to the appliance. Make sure you have attached the dust tray properly to the appliance.

The dust tray may have been detached by accidental pressing of the tray release button. Make sure the dust tray is properly attched to the appliance.

The dusttray may not have been closed properly. Make sure the dust tray is closed properly before it is attached to the appliance.