Catasetum new growth issues

[Maxx]

Hi Everybody,

I am relatively new the Catasetum alliance, with only 3 of them acquired in course of last year (a Fredclarkeara After Dark 'SVO Black Pearl' FCC/AOS; a Monnierara Millennium Magic 'Witchcraft' FCC/AOS, and last but not least a Cycnodes Taiwan Gold 'Orange'). Out of the three only the Fredclarkeara flowered.

After the end of their dormancy, when the first little nubbins of the new growth appeared, I repotted them all according to the PET method (1/3 layer packing peanuts in the reservoir, 1/3 layer medium orchiata bak in the middle, and 1/3 pure sphagnum moss tightly packed on the top) without watering them, and here is what happened:

Cycnodes Taiwan Gold 'Orange' (Pics): This one took rapidly off initially with thick new growth and roots, but after I started watering (when the new growth was about 15 cm in height and several roots have penetrated the medium) some of the roots began to turn black and mushy, and eventually died-off.
When I noticed this, I replaced the top layer pure sphagnum with a sphagnum / orchiata bark mix on all three plants, and added several holes for additional aeration.
Fortunately, some of the roots survived and branched extensively eventually reaching the water reservoir at the bottom of the container. So I guess this one will survive and hopefully bloom for me, although I still see some roots turning black.

Fredclarkeara After Dark (pics): This one started relatively fast to grow a new shoot after it has flowered, but it was very flimsy, the roots very thin compared to the cycnodes, and eventually the new growthy has become soft and mushy and died.
After the first growth died, a second growth started to grow, but this one seems also very thin and weak with small roots, so I am pretty concerned whether, it will make it. Any idea when I should start watering?

Monnierara Millennium Magic (Pics): This one started producing some nice thick roots, but in overall the new growth has not grown too much. Also some of the root ends seem to have dried-off for some reason. Should I fill up the pot with sphagnum to be at the level of the new growth, and begin watering?

Growing conditions: South-eastern facing windowsill, with plenty of direct sunlight from morning to about 2:00 pm, temperature: ca. 22-24 C, humidity is around 40-50% with humidifier).

Sorry for the long text, but I am really curious what I am doing wrong, as I purchased now a bunch of new flowering size catasetums which have all about 10-15 cm of new growth in small nursery size soft plastic pots packed with pure sphagnum, and I do not now whether I should try to repot them (and if yea, in what? PET method, pure sphagnum, small orchiata bark?

Thanks for any help in advance!