Is there an o-ring between the mounting flange and the BOV body? If using an early flange design, is an o-ring installed on the o-ring groove of the flange?
Port C needs to be plumbed to a boost only source between the turbo and throttle body
Port C cannot share the signal with any valve that bleeds, vents or regulates that pressure source
Port C can share the signal with pressure sensors, or boost gauge, or other "dead end" components that don't modify the signal
Is the valve still secure to the actuating piston? Make sure that these two components are still fastened together.
Have you plugged one of the unused ports on the back of the BOV? Remove the plug and allow the unused port to vent to atmosphere.
Is there debris in the small ports on the back of the BOV? Check the signal line and fittings
Has the pre-load spring broken? Remove the 6 bolts from the back of the BOV and check the inside of the BOV. You can't hurt anything by taking the BOV apart, but be careful not to lose any o-rings that are installed.
When you push the valve in with your finger, does it feel sticky or resistant? There should be no resistance, it either needs a re-lube or disassembled to verify that there are no components broken inside
Is your signal line secure? Make sure that your signal line does not have a leak.
Having a solenoid, or Manual Boost Controller sharing the signal line to the back of the BOV will have it open pre-maturely at lower pressures
We have had many customers over the years where their signal line was melted in a section that you can't see with the naked eye, or had cuts in the line. Inspect the line by tracing it by hand and following the line from signal source to valve
In a Port C enabled V1 valve, you can always eliminate it from the equation by removing all signal lines to Port C and A+B and leaving them to atmosphere. This will make the valve operate as though there's nothing installed on the charge pipe