Were you using the exact same MBC with your last turbos? If so, has the adjustment knob/dial been changed at all since then? (Clockwise increases boost, counterclockwise decreases boost. Completely counterclockwise would run wastegate pressure).
If I remember the hallman MBC correctly, the nipple coming straight out of the end goes to your boost source and the one coming out at a right angle goes to your wastegate actuator. The wastegate nipple has a tiny hole on one of the flat sides of the barb that it needs in order to function, so if at any stage you took these fittings out make sure they went back in the same way.
Another possibility could be the wastegate not staying closed properly, perhaps caused by the actuator arm being improperly adjusted. Did you buy this set of stock turbos brand new or second hand? If you try to move the actuator arm with your hand is it hard or does it move easily? I'm assuming the stock turbos are internally gated, otherwise ignore this one as I have no experience with external wastegate setups (yet ;P)
Are you running the standard ECU or aftermarket? Have you checked for any error codes, possibly from blowing your old turbos?
Without knowing your full setup that's all I can really think of... Has anything else been changed between when you had your old ones and now, even something as small as vacuum lines etc?
I'm in no way an expert, just thought I'd put my two cents in until one of the more knowledgeable members can help you out
By the way what boost were you running before?