For that amount of low boost would suggest the amal valve or faulty wiring .

If you are using half hole of preload which most do and you have replaced the amal what kind of actuator are you using ?.If you are using the standard type you need to pressurise it to see if its working.If its after market take it apart and check the diaphragm .My boost dropped in half and the amal was buggered but then the actuator also went.

Could also be the penny I suppose.What does it sound like.Does it boost but feels flat and shit slow or ? ?

Air leak to create a mega drop you should be able to hear as there is something wrong.