That may be your problem. It does lower it, but it's not very stable.
Get some liquid pH Down & you'll be golden!.
I too use Bontanicare Pro Grow & Pro Bloom, and the liquid pH down is very stable with these products.
I change my buckets once a week and my pH is stable for the entire week and this is adding a 1/2 gal of water to each bucket per day. I add one tsp to a gallon of water and then I pour it into my buckets.
Hope this helps!