There are a couple of things that may cause this.
The first would be to change the Ext0 Heat Manager to PID (a value of 1) from the default setting of Deadtime. This can be done using the LCD and going to the Configuration then Extruder settings. Make sure the settings are saved after being changed. Once this is changed you will need to do PID Tuning procedure. PID (Proportional-Integral-Derivative) is a control loop feedback controller used to help machines make decisions. In this case, the decision of when to increase or reduce heat to the extruder. See this page for instructions for Extruder PID Tuning
The second reason could be that the setting for the Max Drive to the heating cartridge may need to be increased. The default value is 240. Try changing it to 255 and re-running the PID Tuning.
The last reason could be because the earlier units shipped with a 6mm heater cartridge but the hole in the heater block it is inserted into is 6.35mm and the heater is not making good surface contact. There are a few methods to fix this from wrapping in aluminum foil to replacing the heater cartridge. See this page for changing out the cartridge.