loosing boost in my impreza wrx?

hi i was wondering if anyone could help me i have an impreza wrx wagon on a 99 t reg it has a front mount inter cooler and a hks mushroom filter and a turbo xs bov i have recently noticed that the turbo boosts but it seems to hold back a lot as if i was loosing boost mid way through i have been told that the bov may be letting out boost so i took it off to see if i could adjust it but i cant i can easily push the piston in with my thumb has anyone any suggestions as to what the problem could be or if the bov piston should be able to be pushed in easily i noticed the decrease in boost before i put the front mount on . having put the front mount on i have increased power but still lack off boost?