You may be seeing the 'swing', where pH in a well balanced system only slowly climbs. This isn't a bad thing by any means, it helps the plants in many ways. Just bring it back down once it's run its course... and 6.2 is as good a time as any.
What is your nutrient changeout interval? I designed my systems to go a specific set time, so I can change and fill and be confident they have what they need to go the interval with minimum add backs. The way to design the system to do this is to size your total RDWC water volume to match the plant mass and the time between changeouts. I do this by measuring how much water I add between changeouts; if the system can handle 50 gallons full, I would consider the batch depleted if I add another 100 gallons of water. If the interval runs out long before this, you're wasting nutes. If you need to add more than double your original water volume, then your total system volume is too small.
To raise ec between changeouts, I add a complete nutrient like CNS 17.
So in my case, i run a 16 day cycle, and so the first interval is 24 days; the girls are smaller so they'll go longer. Second interval is 16 days; early peak bloom. Third interval is late bloom, another 16 days. The last 8 days is flush, for a total of 64. Very predictable, very repeatable- which is exactly the kind of no surprises operation I like!