Are you running an internal wastegate and flapper (like a stock T3 or 16G or something)? If so, and you're SURE there's no exhaust leaks before the turbo, take your wastegate actuator off the flapper valve. Swing the flapper so its all the way closed, and see how the actuator fits on there. You may need to either bend the actuator away from the flapper or carefully bend the ear of the flapper closer. Basically you should have a slight bit of tension in the wastegate actuator to get it onto the flapper assembly. Anything less, and the wastegate is never fully closed and you won't get much boost.

Is this an intercooled setup? Either way, check all of your plumbing to make sure the clamps are VERY tight. Also check all of your vacuum lines. Make sure none are lose or off.

Do you have a boost controller? Bypass that temporarily and take it for a drive to see what happens. If you have the boost controller connected wrong (manual) you may not get any boost.