Did you put them in the pots with hydroton from the start?

You went from once a day watering to 3 times a day?

Sounds like the hydroton is hydrating and when it starts to evaporate the rock wool takes the humidity.

If the water was touching the bottom of the cube 3 times a day then it'd remain alot more wet than it was when you were watering by hand once a day...

Hmmmm.... pH seems fine ..

Maybe some different input from other members will help you solve this problem.

best of luck,
Bob.