This can not be from cold. Temperatures are: 27-32*C at day and 18-25*C at night. Humidity is never lower that 65%.
Now I discovered that it may be soil. My bicalcaratas in sphagnum moss are growing quite good but the ones in mix (peat, bark, coco chips) are all dying. But rafflesiana and ampullarias grow very well in this mix.
Today I have repoted sick bicals to fresh sphagnum + perlite.
It does look like the roots are sick, drying the plant out. I don't know why that soil isn't good for them, but that does seem to be the case.
BTW, 20 C sounds a bit chilly for N. b..
These are some really bad companies, all shells of Plantron Inc., so don't order from them unless you want small and dead plants they don't want to refund:
Farmer Seed and Nursery
Four Seasons Nursery
House of Wesley
Richard Owen Nursery
How do garbage companies like these stay in business, ripping people off year
Dave - thank you for answer. Yes I think that is the case. Besides I think Bicalcaratas was weak when I received them. I have potted them few days ago to sphagnum with perlite. Maybe they will recover but now they look very "tired".
I wonder what was wrong with my mix? Rafflesianas and Ampullarias grow very well in the same mix. But I see that something was not alright because there was small flies when I put plants in this mix to higher humidity...
Dave You say that bicalcarata doesn't like temps under 20*C?
I notice you have coco husk chips in that media. There are varying qualities of husk available and I myself found it out the hard way. I would say that is the problem. See if you can replace the media with a sphagnum moss/perlite/fir bark mix. If you don't have easy access to sphagnum, see if you can get peat moss and appropriately mix it with increasing amount of bark and perlite to keep it relatively open.
Sorry to write so late. Just wanted to tell that these bicalcaratas have recovered. Thanks everyone for help! I have repotted them to my standard mix (NZ Sphagnum + pelite) and they like it very much. Only one have died - it was very sick before repotting.