If your pH never seems to settle, alkalinity is almost always the cause. Here is the relationship explained simply, with the reset routine.
Total alkalinity is the buffer that holds pH steady. When alkalinity is too low, pH bounces around with every dose, every aeration cycle, and every bather session. When alkalinity is right, pH largely takes care of itself. Most pH troubles trace back to alkalinity being out of band.
Alkalinity is the water's resistance to changes in pH. Chemically, it is the carbonate and bicarbonate concentration that absorbs hydrogen ions before they can change the pH.
Think of alkalinity as the cushion. A small dose of acid against a small cushion makes a big change in pH. The same dose against a big cushion makes a tiny change.
The target band for hot tubs is 80 to 120 ppm. At those levels, pH typically holds within 0.2 of where you set it for days at a time.
pH that swings between sessions: high after bathing, low the next morning. That is a classic low-alkalinity pattern.
pH that you can adjust but it returns to the same wrong value within a day. That is the buffer pulling it back to its native equilibrium.
pH that responds wildly to small doses (a teaspoon of pH down moves it half a point). Definitely low alkalinity.
Test alkalinity. If it is below 80 ppm, dose alkalinity increaser (sodium bicarbonate) per the bottle. Wait an hour, retest.
If alkalinity is above 120 ppm, dose pH down (sodium bisulphate) carefully. The same product lowers both pH and alkalinity, but the alkalinity drop comes first if you add it slowly with the jets running.
Once alkalinity is in band, leave pH alone for an hour and then test. It will usually have settled into a comfortable range without further work.
You can but you will waste a lot of product and never get a stable result. Fixing alkalinity once a month is much less work than chasing pH every day.
Above 150 ppm in a hot tub it tends to scale. There is no benefit to going above 120 ppm. The buffer is more than adequate at that level.