check and make sure you don't have the wrong nipples connected on the gmbcs.
at rest, with the car off, you should be able to blow from one connected nipple to the other. the car energizes the solenoid to raise boost. this will stop the flow through the solenoid.
when troubleshooting you can also disconnect the BCS entirely to make sure that your MBC plumbing is proper.
finally make sure that when you're WOT, whatever is controlling your solenoid goes to 100% duty. if you're using an oem ecu to do that, you need 100% wgdc and a boost target that's higher than you'll actually clamp it to with the MBC.