first thought is actuator, i've just replaced my spring in my collins one due to the same problem, it's mint now and holding much more boost, without a bleed valve i could only get 7psi max, now on the actuator alone i'm getting 14psi, open bleed valve half a turn and hitting 16, one and a half turns and it's about 21psi, which is the limit for my ecu's map sensor.