To elaborate on what Tuan said, 90% of PH issues are CO2 exchange issues. If your Alk is in the normal range and you still have low PH increase Oxygen. He was dead on with the open window or pull in air from the outside. I just read that a tank with ferns next to it saw a very healthy PH upward swing due to the plant converting the CO2 to Oxygen.
I would do a few water changes as big as you can and see how things respond. I would then do a test of PH then open a window near the tank for a few hours and retest and see if you see an increase.
I too had PH issues( 7.7-7.8 )but for mine it turned out I had ALK that was like 5.8 DKH. Once I raised it to 8.5-9 DKH , PH is now constantly in the 8.1-8.2 range.
Also, stay away from PH buffer, it's the devil.