Bulbophyllum A-Doribil Anna Roth leaf fading

[BernieK]

I'm at a loss, after considerable research I can't seem to diagnose why the leaves on my Bulbophyllum A-Doribil Anna Roth are fading and loosing their leaves. The plant is grown under LED lights, daytime temperatures range in the mid to high 70's, good air flow and watered weekly using MSU fertilizer with additional calcium added to the mix and monthly flushes with epsom salt flush every other month. The only thing might be humidity, currently levels are low during the day ranging from 40% to 50% with night time levels ranging 50% to 65%. The humidity issue is currently being resolved so I can maintain 60% to 70%. Any help will greatly be appreciated.
Cheers, Bernie