I tell folks to wait at least five or six hours before retesting. Do you wait that long? I generally have a five-hundred gallon tub and our tap water is 8.0 (usually). I put in four ounces of "Spa Down" while the water is cold, then wait six (ish) hours until it is hot. Then I test, and add three or four more ounces, and add my dichlor.
The following day, a test will reveal if I need the last ounce of spa down or not. This works every time - the only variable is that last ounce.
Our water tends to creep up in pH, so I toss in an ounce now and then, mostly in line with how much jet usage (aeration) the water gets.
Also - test strips will not give you a reliable pH reading if your chlorine level is over about 3ppm. Don't even bother if it higher than that.
HTH