Sticky or twisted coffee beans have not slid into the grinder and have caused a blockage.
Clean the bean containers more often if you are using darker, more oily coffee beans.
Try other types of coffee bean which are less oily.
Use a coffee type with smaller or more evenly sized coffee beans.
Release twisted beans from the grinder. To do so, proceed as follows:
Disconnect the appliance from the mains.
To move the coffee beans, use a narrow spoon or similar to stir the beans in the containers.
If the problem occurs frequently, try using less oily coffee beans.