You need to start with PH correct water first of all. Before you add nuts the PH should be correct if Ive read correctly...someone please step in if Im wrong but Im pretty sure that instrustions said that PH needs to be right BEFORE you add any nuts to your water. That could be your problem and why your PH is all over the place I bet....
But to answer your question, yes, some of the plant even same strains may use up something it needed and that missing from the water can change the PH level as nuts are being eaten by each plant.....
But even so, you should still mix in the correct order which is starting with correct PH water right off the bat, then add all the other nuts your gonna add. You should also mix the nuts in the last remaining gallon and mix it well, and THEN add it to the tank (res). Some just dump the nuts right into the tank which is really not the proper way to mix in your nuts to get the most out of them. Liquid stuff is a bit different than powder though I would imagine but I would still follow the same mixing process.
You PH balance the water AFTER the nutes are mixed in. If you did it before you would have a very acidic mixture as the nutes will drive the PH down further.
tools less then 3 weeks oldcalibrate that ph meter,, sounds to me you might have some rez on your tool,,
One thing no one has mentioned yet. You are using small reservoirs. I never use less than 10 gal rez's because the Ph use to fluctuate widly like yours. As the plants drink especially if they have some size to them u will be ph ing every day because the smaler volume of water. Here is what I found out over time...switch to 10 gal rez. Use distilled water and put it in a larger 35 -50 gal. trash can and ph it all at once. If you want u can even ad the nutrients if u want, just make sure u keep it in the dark so no algae will grow. also keep more plain distilled phd water on the side. why? because if u ppm ur nute water to 1000 u can calculate by the volume of the res as to how much to add to get it to wherever you may be (i.e 400ppm). u need clean water to go with it to dilute it. this way saves time and u wont be going crazy ph ing ur water all the time. more water= more stability. Also the more intense the light, the more your plant will drink.