Bet you use Tri-Chlor pucks to chlorinate your pool. They are VERY acid and drive pH down.

Otherwise, MW Smith is right. I ONLY use 20 Mule Team Borax (from the grocery store) to raise my pH. I usually add 1/4 box at a time, wait 12-24 hours then test and add again as needed. However, if my pH is SERIOUSLY low (<6.9), I'll dump in a whole box, wait 6 hours, then test again and add if needed. BTW, my pool is 20k and vinyl lined.