why deos my ph keep going up?

i have a ebb and flow system and the ph stays at around 6.7 when i adjust to 5.5 it takes 2 hrs to go right back up why is this? this is my second grow in this system but i cleaned the hell outa the system. temp is normal 68 - 70. the flood peroids are 4 hours apart and there is a little water in the bottom of each bucket for 4 hours could this be the reason?
any help much appreciated thanks

The ph down dissipates (evapourates). I generally ph down before I feed because of that.

If it's after a watering you could notice higher ph due to the nutes or micro nutes your plant is taking in.