Although you didn't list pH change, it appears your plants need a little leaner mix. I don't know where I found it, but the following helps me understand what's happening when there are changes in my res:
PPM goes UP and PH goes DOWN = Plants require LESS nutes.
PPM goes DOWN and PH goes UP = Plants require MORE nutes
PPM stable and PH goes UP = Good thing.
Good luck with your grow!