I did get it! (many thanks - smelly stuff that).
I've got a few clues that are leading me to humidity - something that has not been a high priority for me (why can a thin-leafed plant like U. endresii be doing well and a thick leaf like U. quelchii be stunned?)
- recent H. 'Tequila' arrivals have struggled (unlike all other smaller Heli's)
- tiny sprig of U. quelchii spread through a small pot all summer but stopped growing & died back when I placed it in the 75 gal Orchidioides tank
- - - same sprig of U. quelchii started sprouting new growth when covered with humidity dome
I've been reluctant to add any fans & humidification but recent events have me pondering that decision ....
Remind me again how far the media surface is (U. quelchii pot) from your lights?