I had a similiar problem except I would build boost sometimes and sometimes it didn't at all. It turned out to be a misfire and knock sensor. Fixed those and haven't had a problem since.

 

It could be your MAF. Try unplugging it and see if the car boosts. I had similar symptoms whem mine went out becasue the A/F mixture ratios get thrown off so the car runs way to rich/lean so you are probably boosting at 5psi but aren't feeling it because 1. you are in limp mode and 2. theres bad combustion so it makes less power then it normally would under boost.