Nutrient solution changes as plants grow. Plants take up water faster than nutrients, so the solution becomes more concentrated, and the pH moves away from where you set it.
Why it changes
As roots absorb water, what remains becomes stronger in nutrients. Plants also take up some nutrients faster than others, which shifts the balance.
Topping up
Top up with plain water when the level drops, then check the pH. Adding only nutrient solution can make the mix too strong over time.
Replacing the solution
Replace the whole solution on the schedule that suits your system and your plants. Rinse the reservoir before refilling. Some growers replace solution regularly; others replace it when it looks cloudy or smells.