5.2 to 6.0 is a safe Ph range for soil less grow mix. Yes there are strains that prefer a higher Ph then the normal range of Hydro Ph but not many of them.

5.5 to 5.8 is the target number you are looking for in Ph.

Did you do a proper Ph balance with your grow cubes before you started?
(Soak them in Ph adjusted water of 5.2 for 24 hours before you used them)