Although the following video is US-orientated, the description of the governor/unloader valve (from 5:45 on) should explain how it works:-
http://www.youtube.com/watch?v=2U_R46CZnUM
I cannot see how a faulty unloader/governor valve would result in inadequate system pressure - unless the spring-setting has been tampered with.
More likely that the system pressure would rise too high as the unloader valve failed to signal the unload.
You could check whether the compressor is working adequately by
temporarily disconnecting the unloader valve from the compressor (probably a small diameter plastic signal pipe unless the valve is fastened directly onto the compressor cylinder head) when the system pressure should rise above the governed pressure until the safety valve blows off.
Check for leaks that might be preventing the system from reaching governed pressure - or the compressor might be faulty . . .
.