I just started using mini GRODAN Grow-Cubes and the pH is dropping rapidly. What am I doing wrong? (The plant is flowering.)

You will see a drop in pH for many plants as they begin flowering. It simply shows that the plant is eating different food than during leaf-setting. If you measure pH in the drain water on a regular basis, you can see the pH drop starts before you see flowers on the plant. It is a signal to you that you should change to your bloom-nutrient. The pH might drop so much that you will need to use some ‘pH-up solution’.