you NEED to have an amal valve or bleed valve on it to make more the 1-1,2bar of boost, as said earlyer the amal valve or bleed valve takes some of the pressur from the actuator and that causes the turbo to make more boost.
It it still wont make more then 20psi, then either the turbo is crap or there is a MAJOR boost leak some where.
Exh. manifold doesen`t mean THAT much tho.
Could also be if u get so much knock that the ecu retards the ignition so it WONT make any boost?