Is this a new install or a new problem? There are a couple of scenarios I can see that would prevent the boost by-pass from closing. First would be no vacuum source or your vacuum source has a leak. Second is the by-pass needs to be adjusted because it doesn't allow the valve to close when vacuum drops to zero. The vacuum motor is mounted to the supercharger on a slotted bracket to allow adjustment. If the bolts backed off or the bracket slipped out of place, you probably won't be able to build boost or at least full boost.
My suggestion would be to study your instructions and check the vacuum routing. On a new install, you might not have routed the vacuum lines properly with the boost control solenoid. You should be able to push the linkage on the vacuum motor with your finger and when you let it loose, you should be able to hear it snap shut, which means the butterfly valve is closing. Unless the valve is closed, you can't make boost. Also try moving the vacuum motor it to see if it's loose. The vacuum motor is located at the rear and base of the supercharger the driver's side.
2008 F-150 Lariat SuperCharged SuperCrew 4x4