You didn't mention if this was a concrete pool - is it a new finish - did you just have the finish acid washed? That may make your PH rise considerabely everyday until the finish has cured.

How long are you waiting from the time you add the acid until you are testing the PH again - is it the next day, 10 minutes, an hour? I add acid with the pump on and then brush the whole pool - it takes about 20 minutes and helps to stir things up - then I retest.

Also how large is your pool - I find with ALK of 80 in my pool - it takes about 2 cups to go from 7.8 to 7.4. The larger the change in PH - the more it will take - and the larger the pool the more it will take.

I would refrain from adding a whole gallon at once - I wait a day between additions just to verify a reading wasn't bad due to the pool not being mixed up yet.