The inlet valve does not open fully

Hi. I have spent a lot of time trying to figure out the root cause to my compressor problem, but without luck. So I´m now turning to this forum for help. The inlet valve does not open fully when magnetic valve activates. It sort of hesitates and this results in very little or no air supply under 4 BAR, over this pressure it works fine. I have changed the wear parts in the loading valve assembly and checked the function by using an external pressurized air supply, and it opens fully when pilot air supply to loading valve exceeds 4 bar. So there is no fault to magnetic valve or loading valve assembly as far as I see. I kind of think that the pilote pressure needed to open the intake valve is to low? This pressure pipe is taken directly from the secondary side of the screws. To build more internal pilote pressure I also tried to increased the springforce on the check valve on outgoing air from the compressor unit, but without any improvements. Oil level is OK and I have tested both lower & higher levels without any noticeable difference. The compressor is pumping fine if I force open the intake valve. I bought and rebuilt compressor unit a with new screw bearings a few years ago and it worked fine for two years before this loading valve problems began to be a big problem. I would be grateful for some ideas as I´m at the end of the road here. Thanks in advance Thomas, in northern part of Sweden.