I have a hard time growing P. holtumii also SHOW us all how to do it. I don't think they like our cold nights here. Wanna do they dry out like P. wallichii in your summers? NO rain? 
when summer season comes I stop water them and let them sleep for 2-3 months. actually in the nature be like that and rain will wake them up in nature.
Wanna, how about those in your nursery?
Do you do the same, as in stop watering?
I would think those in your nursery is in a more
Controlled environment so maybe no summer?