When the EC drops and ph rises then that is a good thing. It means the plants are happy and taking up nutrients. It also tells you can feed them a little more. Just make sure you have plenty of O2 in the resevoir or bubbler. The more O2 the better uptake of nutrients. I change my resevoir every Saturday. The second saturday I do a flush before that res change.

Did you check the flush water for ph and ec? I was wondering how much salts was locked up in the medium.