Also is that cabin Altitude climbs as we go above 28000 ft but the differential pressure stays at 7.45 psi it should go up to 7.8 psi

Problem On a Boeing 737-300,400
Climbing to 27000 ft the pressurization system works fine7.45 psi, on the cross over to 28000 feet the system falls behind and seems to climb as the aircraft climbs pressure is above 7.8 and higher.

We have changed the controller panel, pressure controller and the Delta P no difference


Please advise