Could be a boost leak (vacuum lines as mention above). But it also might be your internal wastegate does not have enough pre-load in it. Some installed the ATP kit and don't check the wastegate to see where it's set.
You can perform a smoke test. After my install I didn't secure one of the vacuum lines properly and it developed a hole, which then leaked out all the vacuum pressure. Without that the waste gate won't open at all. A smoke test is basically a machine that fills the crankcase with smoke. The smoke will exit from any point that's open, so any vacuum line that's leaking will smoke. Took mine to a performance shop and found the leak in 30 seconds, drove home and fixed it. If you do this just make sure you have the tray under the motor removed so you can see the turbo and vacuum lines. Hope this helps.