You have a leak. No mine stays solid. Those little valves can leak, pipe connection can leak. The pumps all leak but a good one way valve shouldnt.
(28mHg) arms on armrests averaged although fairly consistent.
(31mmHg) arms lifted up also averaged and rounded to full digits.
Acceptable limits of peak pressure (generally over the ischial tuberosities) are 40-80mmHg if pressure is regularly relieved. Every 15 to 20 mins. According to many studies. People vary. This is from mapping via a pressure mapping services and lots of studies.
Safe pressure is around 27 to 32 mmHg without damage in several online pressure mapping clinical trials. And this is hard to achieve. Only the deep roho seems able to match this figure regularly with careful setup.
Considering that in a roho the pressure is always equal throughout the air cushion then the actual measured pressure is = to the pressure on your backside or a pressure mapping result.
It would seem that you are either on a cushion that is too big for your body, hence higher pressure needed, or/and you are not immersed adequately. Go for 10mm finger thickness. And you got to find the leak. Soapy water and a brush.