All is ready in P3D4 on client, and AS16 & P2A are running on server. Jetmax737 has been installed by Sim-Avionics. ATIS information includes QNH. On the PFD, the yellow marker shows above or below the default 1013 (29.92) if the announced pressure is different.  It aligns when the ATIS announced pressure has been set by the ECU.

Occasionally the entry on the top line of P2A Control Panel matches, but mostly it does not.

Any ideas on what I could be doing wrong?  Other detail along that line changes to match settings.

 

The Altimeter on the top line of P2A is not the current altimeter.  It is the Altimeter Setting.  Click on it and set it to the altimeter being reported by ATIS.  Or, if you set the altimeter in the cockpit, it should match what is set there.

If you are setting the altimeter in the cockpit, it is possible, though rare, that the aircraft does not implement that setting in a standard fashion so FSUIPC does not report it.  In that case, there could be a mismatch.