See p.9 of the HD621 manual here.

Set the type in MConfig according to your setup (not sure from your signature whether you’re running a centre channel or not).

Try that after you’ve reset it and see if it just works.

If not, in the sources tab, check you haven’t got something like Last Valid in there for each source, so actually assign the correct source key per real input you’re using and make sure they correspond to the sources you have set up in the Master speaker.

HC: 861v4/ID40/LPS, DSP5000, DSP5000VC, DSP3100, SW1600, HD621, Oppo BDP-105, Sony VPL-HW55ES
2-Channel: Oppo BDP-105, TEAC VRDS-7/Trichord Clock 3/PSU, 551, Quad 303, Harbeth Compact 7ES-3, Kimber cables, Russ Andrews mains and supports.
Previous Meridian: 5000C, 5200SL, 7200.2, 6000, 562V.2, 568.2mm, 800v4, 808.2i, 818v2/v3, 861v8, 502