Cattleya walkeriana 2 out of 3 buds turned yellow

[dial15]

I have had Cattleya walkeriana v. semi-alba x sib (Baby's Tears x Elixir of love) about 6 months and it produced a flower spike from the base of a pseudobulb with 3 buds. The first 2 had turned yellow and stopped growing while I was away for 5 days. The terminal bud is alive and swelling. It looks a bit like a flamingo's beak. There was the usual sweet sap on the buds. the plant is healthy, producing new roots. Its in chunky bark in terracotta with extra holes drilled on the side of the pot. I generally run water with dilute fertilizer over the roots every 2 or 3 days so I left a little water in the tray for humidity while I was gone. I have read all the walkeriana posts because I have another walkeriana x intermedia 'Midnight blue' for over a year. It's growing well, producing pseudobulbs and roots but no buds. I give it complete darkness at night.
If anyone has experience to share, please do.