Did you soak your rockwool in PH balanced water first?? Rockwool is high in PH naturally and so it is always wise to PH balance the rockwool cube by soaking it in PH balanced water for 24 hours before you start using it.

Also, I would say that 6.5 is still a bit high...I would go for more like 5.8-6.0 but that is just what works for me...

Can you give more details?? Setup? Lighting, temps humidity etc?? That might help out a bit more too...

Also, when you PH balance the water and you use a product like PH Up or down, the PH up and down with gas off for the first few days causing the PH to fluctuate...I would say it is a number of factors but knowing more about your setup would help everyone better diagnose your problem!

Good luck!!