I don't know for sure, so here are some conjectures.
What is the day length when it's under lights? Too long a day length in winter might also inhibit flowering. I would try to give no more than 10-11 hours of light in winter.
I know this may be hard for you to believe, but it looks like it's not getting enough light intensity. Midsummer sun in northern Europe is much dimmer than close to the Equator. I don't know where it was originally grown. Perhaps they used supplemental lighting all year. If you go to orchidspecies.com and look up Den. discolor you can read this:
This plant if not blooming needs more light. In nature it always grows in exposed high light conditions.