First thing is always to run a temperature monitor like in repetier host/server and enable output view. See if it tries heating full time full power or if there is room for more heater power.
What often happens is that a fan starts cooling the heater and heater has not enough power to overcome this cooling. Or it cools but calibration was done with fan off not considering this important factor.
A good designed extruder should have a fan for cooling extruder if required and one for cooling the print and have not much effect on the heater block. I know that is easy to say and hard to accomplish so some mixing is always present and to some extend this can be compensated. But too much can cause that problem.
