When your machine is not brewing, there is too much pressure in the group head due to one or more of the following reasons:

1. The filter basket may be clogged. Remove the basket from the filter holder then remove the filter plate from the basket. Rinse under running water to ensure all the holes are open. To unclogged holes use a small pin or needle.  
   
2. There is too much ground coffee in  the filter. To fix this, you must ensure the amount of coffee in the filter holder does not exceed the line level in the filter basket.Try turning the ground coffee dial to deliver less coffee but maintain the coffee level.
3. The coffee was ground too fine and the water cannot push through the grounds. In order to fix this, you must adjust the grinder setting to a more coarse grinds.