Winter watering and fertilising for struggling bulbophyllums

[AussieGrower123]

I've got a small collection of bulbophyllums and I'm a bit unsure of what to do with them coming into winter regarding watering and fertilising. Some are recent additions and so I've not seen how they behave in colder weather before. I've been watering and fertilising more frequently over the past months and some seem quite happy with the treatment. These plants are smallish - much bigger that tube stock but under about 20cm across. Do bulbophyllums need resting over winter? If yes, how frequently would you water and fertilise?

On top of that, some are struggling - the bulbo ambrosia seems generally unhappy (no new growth and the roots look tired and brown) and I'm having significant trouble trying to plump up the bulbs on the bulbo guedoreii - I've been trying to coddle both with weak fertiliser each watering (about every one to three days depending on how moist they seem) but they're just not taking the hint. The guedoreii had a new growth recently but that's gone dark and died off. Not sure what I'm doing wrong with these - my bulbo scaberulum and bulbo roxburghii get the same light and even less water (though they are larger plants) and they're flourishing (seriously - the roxburghii just put up about 8 new leaves and grew by maybe a half what it was before).

I've also got some tiny bulbos that look even sadder - they're in what appears to be thick moss and I suspect they're getting too much moisture - which is odd, as the seller I bought them from said they were several years old and flowered already, implying they'd been liking that medium in the past.

Any suggestions for what can be done to encourage/ rescue these plants would be much appreciated!