Regarding the change in cabin pressure differential with no change in engine power during descent, my guess would be there is a change of sensed differential by the outflow valve.  I don’t know which outflow valve and controller you have.  The Duke’s valve and controller is fairly simple: the controller drives a small DC motor in the outflow valve to set the target differential pressure by moving a screw that applies more or less force on a spring and diaphragm combination that balances against the pressure differential, thus controlling the outflow.  When you begin the descent, there may be a local increase in the airflow around the outlet side of the outflow valve.  The outflow valve senses this rise in pressure and opens in response to release more cabin pressure.

In regards to not being able to achieve maximum cabin pressure differential, it is simply supply versus sum of controlled and uncontrolled release.  Make sure there are no leaks in the supply-side plumbing by capping off the supply duct into the cabin.  Cap all but one of the pick-offs from the engine induction plumbing and supply about 6 psi of regulated air into the engine-side scat tubes and mixing valve.  Use a stethoscope with the sense end removed to probe around the plumbing to hear the air leaks.  I sometimes find the wire in the scat tubes has cut through the rubber fabric due to clamping pressure and will leak there.  I will sometimes find leaks along the length of the scat tubes where is secured by clamps or chafes against something.

To find the leaks in the airframe, pressurize the plane on the ground by supplying high volume, low pressure air via the mixing valve and capping the other hot or cold source.  I have used the pressure carts from shops that service jets, large shop air compressors, and rental diesel-driven field air compressors, such as used to drive pneumatic jack hammers.  Be sure you have some method of determining the differential pressure and controlling the supply.  I suggest removing the interior cabin panels and seats first so you have access to all areas it can leak, such as the forward and rear pressure bulkheads, the gearbox, and all the pass-throughs for flight controls, wiring and plumbing.  Set the cabin pressure controller to the lowest cabin altitude. Have one person in the plane while it is pressurized with the stethoscope and listen for air leaks.  You may be surprised at how many leaks you will find.

Once I had sealed all the leaks in my plane, it will maintain 5 psi differential cabin pressure at absolute manifold pressure as low as 22 inches of mercury, regardless of altitude.…Jeff.