Cp563e won’t vib on low

Hi all I have a cat cp563E and having problem with low vib. It will kick in when cold but when machine warms up it won't! Maybe only few times. High side works fine I've checked charge pressure that's ok. I checked for pressure on the servo ports it pressures up when I hit the button but doesn't vib?? The solenoid vales have been changed btw I've been told could be case pressure or servo problem Any help would be greatly appreciated Regards Michael.