Progress with Paph seedlings - problem with one

[shakkai]

I purchased 5 Paph seedlings back in February to see if I could grow them - I didn't think I had the right environment for them (too cool) but thought that I could possibly acclimatise seedlings to the cooler conditions.

Some seem to be doing well, another is okay, but one is distinctly worse off than the rest. The one that is not growing well is the only one that is a pure species (Paph. hirsutissimum) the other are a mixture of hybrids.

Here is a photo of when I got them:


Here is a similar photo of them today:


Both of the mottled leaved Paphs seem to be happy. St Swithin have increased quite a lot in size. The two that were having problems (which have both been transferred to grow in Rockwool after they started to show 'issues' about two months ago) are the species hirsutissimum and a cross of rothschildianum x gardineri (syn. glanduliferum). The rothschildianum x gardineri has begun to recover, and is now putting out a new leaf (after losing two before I repotted), however, the hirsutissimum is just sat there.

Is there something special I can do or try to coax this one along? The roots (all two seedling roots) are still brown, hairy, and firm.

Many thanks in advance!!