2 things come to mind.
One is you need to have the correct messages enabled in the GPS.
Two, and this is very subtle, is you have to have the spacing set correctly for the gps_mb_ofs_* parameters. It seems if ardupilot calcs something different based on the distance given from the gps it just won’t work without warning. This took me a long time to figure out - admittedly from reading GPS for Yaw (aka Moving Baseline) — Rover documentation
“that the reported distance between the two modules matches the distance given by the GPS position parameters within 20%”