DWC Ph and PPM dropping?

I am running DWC and I am 30 days into veg now. The ph and ppm keeps dropping I know if your ppm drops and Ph goes up or stable you add more nutes but what do you do if ph and ppm drops.

Yesterday I gave 500ppm and ph was 5.9 - 6.0 this morning I checked and was 450ppm and ph 5.5 - 5.6 what does that mean?

Odd that Ph and PPM are not more stable in Veg. Are you running organic or synthetic nutes? tap or RO water? Are your meters properly calibrated? I understand DWC Veg should run at 5.8 Ph.

For better response in this forum give more info. Post pics if able.