problem with vacuum in turbo

My turbo isn't forcing the air its supposed to into the engine. Im not getting the boost its supposed to have. i just replaced the original turbo becuase it was leaking oil. I put the new one in and there is no boost at all. What could be the problem?

What car. What turbo. Are you using the stock guage or aftermarket. What mods like aftermarket wastegates and the like. We need some info if you want some help ;)