Open more windows. Sometimes there is not enough air moving towards the shutters and the louvers cannot open all the way. Another thing to try is putting felt strips or rubber stripping on each louver to give the louvers a little weight and catch the vibration from the whole-house fan. Check to make sure there is enough clearance above the fan. A minimum of 30" of clearance above the whole-house fan blades is needed. Without this clearance, the air will be forced back through the shutter. Also check to make sure there is enough “exhaust” ventilation, which includes both intake vents at the roof’s edge or in the overhang or undereave as well as exhaust vents high on the roof at or near the ridge. Without enough “exhaust” ventilation, air will also be forced back through the shutter. It might be necessary to add a 2" x 4" to the housing of the fan to give more clearance between the fan and the shutter.