I've had the same problem. Low boost was easy to set and very consistent by high setting was just a paiiinnnn...
However, i did manage to get it to the boost i want..it just gets there inconsistently.
I'm aiming for 1.3 bar high boost and my actuator is a 0.8 bar ...So it shouldn't be a problem. I've checked everything related to wastegate but nothing was wrong..so i just left it at that
First thing i'd do is check for any boost leak. Check the pipes that go to the wastegate and also the housing. If thats ok, check the relation of wastegate opening rate and actuator arm pushing it. If you are not achieving the boost you want ( lower than what you are aiming for ) , then your wastegate is probably at its full aperture too early.