When did it last work?
Pretty much something is restricting how much the actuator can open the gate - so in my opinion it's either boost controller (which you don't have), actuator itself, actuator rod preload, or gate fouling on something. Well also, could be that the internal gate simply isn't big enough, but I doubt that.
If you can get to the turbo/actuator while it's on the car, I'd recommend disconnecting the actuator rod from the wastegate. Once it's off, ensure that the wastegate can open and close freely. Also ensure that when you are putting the actuator rod back onto the wastegate, you need to pull the rod out against the spring roughly 3mm. If any more, then overboost is possible.