You need draw from the right locations to move heat out. If you use an attic fan, you should seal your gable vents and have no ridge vents only soffit vents. This will create draw from the bottom moving heat out. Your attic fan should be near the peak. If you put an attic fan in without blocking the gable, and have a ridge vent, the air will pull from the gable or the ridge vent but the doctor vent. You will not be moving the heat away from the inside ceiling of the living space of the house.
Also you need velocity. Drawing from the larger gable vents you are not creating velocity. It's best to draw from a smaller area to create velocity, which the soffit vents are.