Many of the insulation installers in this area do not use baffles in the soffit area to allow air to come into the attic. Think of your attic as the emphysema man...he can not get air out of his lungs if he can not get air into his lungs. There are a couple of solutions I know of. Install baffles which is not a fun job after the insulation is already in place. Another alternative exists if your home has gable ends. You can install a solar gable vent fan on one end bringing air into the attic and another solar gable vent fan blowing out. The two fans working together will usually create enough air flow to provide the proper exchange of air. The last option is too plug off about eight feet of the ridge vents in the immediate area of the fan. When a fan is installed to close to a ridge vent or static vent, the fan takes the path of least resistance and brings the air supply in from the ridge vent instead of the soffit vents. Then the fan will short circuit and suck in the air that has just been exhausted from the fan. This creates a circular air supply so all you are doing using the hot air you just exhausted to try and cool. Meanwhile the hot air will stack up to as high as 150F. To prevent this from occurring, a fan should never be installed less than five feet from a ridge vent unless it is physically blocked off.
Also, once you get the air supply issue solved, it would be helpful to properly insulate the attic stairway opening. Without insulation, this stairway functions as a chimney and just like a well designed fireplace can create quite a suction as the hot air rises up into the attic. Remember hot air always rises. I would be happy to conduct an energy balance study at no cost to you if you are interested. PM me or email
jp@jphomeservices.com