more than likely the problem is the gauge, hopefully it is calibrate issue.
and what is the temperature of the co2 tank?
if the temperature is low(not the frosted freshly filled), the pressure is low too.
I remember last winter there was a member his room temp is about 60, the pressure is around 600 psi, but back up to 800 psi when the room temp rised to 75 for a couple days.
Please listen Betta tells it like it is no matter what. No bull pucky. I would of used another adjective but this forum does not allow it.