Double check to make sure the hose you're using has sufficient wall thickness. If you use to thin of a wall hose, it could suck closed under vacuum... basically collapse on its ID and then you get min to zero vacuum pull on the modulator.
On another note, if you used an adjustable modulator valve, you can adjust it's performance at the small set screw located in the vacuum port. A very short, thin blade, straight screwdriver is needed - little bit tight in that area sometimes.
Last, double check to make sure the pin was installed first into the port where you placed the modulator.
DJs74