Check the intake ventilation. Make sure that the attic insulation has not covered the intake vents preventing air from feeding the power vent. It might be necessary to either insert insulation baffles or to pull the attic insulation back to ensure the intake vents are not blocked.

Also check to make sure that the plywood the intake vents are installed in have actually been cut or opened to allow for air to flow into the vent.

Finally, make sure that the power vent has enough intake vents. To determine how many intake vents the power vent requires, take the CFM (cubic feet of air moved per minute) of the vent and divide it by 300. This will determine the square feet of intake ventilation needed. Take that number and convert it to square inches by multiplying by 144. For example, 1500 CFM divided by 300 = 5 square feet of intake ventilation needed. Converting to square inches looks like this: 5 x 144 = 720 square inches of intake ventilation needed for the power vent.

If you don't know the CFM of the power vent or the net free area of the intake vents, check with the manufacturer.