How can I fix my pressure loss?

You have a leak. Could be a faulty valve, leaks only when open, or closed, So depending on ambient temp, will only open/close sometimes. IF you have thermostatic control valves, take off the head of each one see if any moisture around the top of the valve, or a brown staining. Just a bit of leak, would dry out as it leaked if radiator hot, so might never feel damp or wet. Also check bleed valves etc on top of radiators.
BUT you have a leak, That is the cause, Now find it.

SD
November 2012